Autoreply: Remove Exim version 3 campatibility code

[exim.git] / doc / doc-docbook / spec.xfpt

diff --git a/doc/doc-docbook/spec.xfpt b/doc/doc-docbook/spec.xfpt
index f856af346219587ee3183e046472cf55bb70e02a..d030a8411105d09657b1964b00c3fa5a0045d72e 100644 (file)
--- a/doc/doc-docbook/spec.xfpt
+++ b/doc/doc-docbook/spec.xfpt
@@ -6742,11 +6742,11 @@ Exim variables you need to construct the database query.
 
 For the string-expansion kind of lookups, the query is given in the first
 bracketed argument of the &${lookup ...}$& expansion.
 
 For the string-expansion kind of lookups, the query is given in the first
 bracketed argument of the &${lookup ...}$& expansion.

-For the list-argument kind of lookup the quury is given by the remainder of the
+For the list-argument kind of lookup the query is given by the remainder of the

 list item after the first semicolon.
 
 .cindex "tainted data" "quoting for lookups"
 list item after the first semicolon.
 
 .cindex "tainted data" "quoting for lookups"

-If tainted data is used in the query then it should be quuted by
+If tainted data is used in the query then it should be quoted by

 using the &*${quote_*&<&'lookup-type'&>&*:*&<&'string'&>&*}*& expansion operator
 appropriate for the lookup.
 .endlist
 using the &*${quote_*&<&'lookup-type'&>&*:*&<&'string'&>&*}*& expansion operator
 appropriate for the lookup.
 .endlist


@@ -8156,13 +8156,20 @@ option, you can still update it by a query of this form:
 ${lookup pgsql,servers=master/db/name/pw {UPDATE ...} }
 .endd
 
 ${lookup pgsql,servers=master/db/name/pw {UPDATE ...} }
 .endd
 

-An older syntax places the servers specification before the query,
+.new
+A now-deprecated syntax places the servers specification before the query,

 semicolon separated:
 .code
 ${lookup mysql{servers=master; UPDATE ...} }
 .endd
 semicolon separated:
 .code
 ${lookup mysql{servers=master; UPDATE ...} }
 .endd

-The new version avoids potential issues with tainted
-arguments in the query, for explicit expansion.
+The new version avoids issues with tainted
+arguments explicitly expanded as part of the query.
+The entire string within the braces becomes tainted,
+including the server sepcification - which is not permissible.
+If the older sytax is used, a warning message will be logged.
+This syntax will be removed in a future release.
+.wen
+

 &*Note*&: server specifications in list-style lookups are still problematic.
 
 
 &*Note*&: server specifications in list-style lookups are still problematic.
 
 


@@ -8339,6 +8346,9 @@ type of match and is given below as the &*value*& information.
 .section "Expansion of lists" "SECTlistexpand"
 .cindex "expansion" "of lists"
 Each list is expanded as a single string before it is used.
 .section "Expansion of lists" "SECTlistexpand"
 .cindex "expansion" "of lists"
 Each list is expanded as a single string before it is used.

+.cindex "tainted data" tracking
+&*Note*&: As a result, if any componend was tainted then the
+entire result string becomes tainted.

 
 &'Exception: the router headers_remove option, where list-item
 splitting is done before string-expansion.'&
 
 &'Exception: the router headers_remove option, where list-item
 splitting is done before string-expansion.'&


@@ -9534,6 +9544,9 @@ start of a portion of the string that is interpreted and replaced as described
 below in section &<<SECTexpansionitems>>& onwards. Backslash is used as an
 escape character, as described in the following section.
 
 below in section &<<SECTexpansionitems>>& onwards. Backslash is used as an
 escape character, as described in the following section.
 

+.cindex "tainted data" tracking
+If any porttion of the result string is tainted, the entire result is.
+

 Whether a string is expanded depends upon the context.  Usually this is solely
 dependent upon the option for which a value is sought; in this documentation,
 options for which string expansion is performed are marked with &dagger; after
 Whether a string is expanded depends upon the context.  Usually this is solely
 dependent upon the option for which a value is sought; in this documentation,
 options for which string expansion is performed are marked with &dagger; after


@@ -14959,6 +14972,7 @@ See also the &'Policy controls'& section above.
 .row &%dsn_advertise_hosts%&         "advertise DSN extensions to these hosts"
 .row &%ignore_fromline_hosts%&       "allow &""From ""& from these hosts"
 .row &%ignore_fromline_local%&       "allow &""From ""& from local SMTP"
 .row &%dsn_advertise_hosts%&         "advertise DSN extensions to these hosts"
 .row &%ignore_fromline_hosts%&       "allow &""From ""& from these hosts"
 .row &%ignore_fromline_local%&       "allow &""From ""& from local SMTP"

+.row &%limits_advertise_hosts%&      "advertise LIMITS to these hosts"

 .row &%pipelining_advertise_hosts%&  "advertise pipelining to these hosts"
 .row &%pipelining_connect_advertise_hosts%& "advertise pipelining to these hosts"
 .row &%prdr_enable%&                 "advertise PRDR to all hosts"
 .row &%pipelining_advertise_hosts%&  "advertise pipelining to these hosts"
 .row &%pipelining_connect_advertise_hosts%& "advertise pipelining to these hosts"
 .row &%prdr_enable%&                 "advertise PRDR to all hosts"


@@ -15706,9 +15720,9 @@ the ACL once for each signature in the message.
 See section &<<SECDKIMVFY>>&.
 
 
 See section &<<SECDKIMVFY>>&.
 
 

-.option dmarc_forensic_sender main string&!! unset
-.option dmarc_history_file main string unset
-.option dmarc_tld_file main string unset
+.option dmarc_forensic_sender main string&!! unset &&&
+        dmarc_history_file main string unset &&&
+        dmarc_tld_file main string unset

 .cindex DMARC "main section options"
 These options control DMARC processing.
 See section &<<SECDMARC>>& for details.
 .cindex DMARC "main section options"
 These options control DMARC processing.
 See section &<<SECDMARC>>& for details.


@@ -16543,6 +16557,18 @@ has been built with LDAP support.
 
 
 
 
 
 

+.new
+.option limits_advertise_hosts main "host list&!!" *
+.cindex LIMITS "suppressing advertising"
+.cindex "ESMTP extensions" LIMITS
+This option can be used to suppress the advertisement of the SMTP
+LIMITS extension (RFC 9422) to specific hosts.
+If permitted, Exim as a servier will advertise in the EHLO response
+the limit for RCPT commands set by the &%recipients_max%& option (if it is set)
+and the limit for MAIL commands set by the &%smtp_accept_max_per_connection%&
+option.
+,wen
+

 .option local_from_check main boolean true
 .cindex "&'Sender:'& header line" "disabling addition of"
 .cindex "&'From:'& header line" "disabling checking of"
 .option local_from_check main boolean true
 .cindex "&'Sender:'& header line" "disabling addition of"
 .cindex "&'From:'& header line" "disabling checking of"


@@ -16640,15 +16666,21 @@ See also the ACL modifier &`control = suppress_local_fixups`&. Section
 .option localhost_number main string&!! unset
 .cindex "host" "locally unique number for"
 .cindex "message ids" "with multiple hosts"
 .option localhost_number main string&!! unset
 .cindex "host" "locally unique number for"
 .cindex "message ids" "with multiple hosts"

+.cindex multiple "systems sharing a spool"
+.cindex "multiple hosts" "sharing a spool"
+.cindex "shared spool directory"
+.cindex "spool directory" sharing

 .vindex "&$localhost_number$&"
 Exim's message ids are normally unique only within the local host. If
 .vindex "&$localhost_number$&"
 Exim's message ids are normally unique only within the local host. If

-uniqueness among a set of hosts is required, each host must set a different
+uniqueness among a set of hosts is required
+(eg. because they share a spool directory),
+each host must set a different

 value for the &%localhost_number%& option. The string is expanded immediately
 after reading the configuration file (so that a number can be computed from the
 host name, for example) and the result of the expansion must be a number in the
 range 0&--16 (or 0&--10 on operating systems with case-insensitive file
 systems). This is available in subsequent string expansions via the variable
 value for the &%localhost_number%& option. The string is expanded immediately
 after reading the configuration file (so that a number can be computed from the
 host name, for example) and the result of the expansion must be a number in the
 range 0&--16 (or 0&--10 on operating systems with case-insensitive file
 systems). This is available in subsequent string expansions via the variable

-&$localhost_number$&. When &%localhost_number is set%&, the final two
+&$localhost_number$&. When &%localhost_number%& is set, the final four

 characters of the message id, instead of just being a fractional part of the
 time, are computed from the time and the local host number as described in
 section &<<SECTmessiden>>&.
 characters of the message id, instead of just being a fractional part of the
 time, are computed from the time and the local host number as described in
 section &<<SECTmessiden>>&.


@@ -17440,16 +17472,26 @@ or if the message was submitted locally (not using TCP/IP), and the &%-bnq%&
 option was not set.
 
 
 option was not set.
 
 

-.option recipients_max main integer 50000
+.option recipients_max main integer&!! 50000

 .cindex "limit" "number of recipients"
 .cindex "recipient" "maximum number"
 .cindex "limit" "number of recipients"
 .cindex "recipient" "maximum number"

-If this option is set greater than zero, it specifies the maximum number of
+If the value resulting from expanding this option
+is set greater than zero, it specifies the maximum number of

 original recipients for any message. Additional recipients that are generated
 by aliasing or forwarding do not count. SMTP messages get a 452 response for
 all recipients over the limit; earlier recipients are delivered as normal.
 Non-SMTP messages with too many recipients are failed, and no deliveries are
 done.
 
 original recipients for any message. Additional recipients that are generated
 by aliasing or forwarding do not count. SMTP messages get a 452 response for
 all recipients over the limit; earlier recipients are delivered as normal.
 Non-SMTP messages with too many recipients are failed, and no deliveries are
 done.
 

+.new
+For SMTP message the expansion is done after the connection is
+accepted (but before any SMTP conversation) and may depend on
+the IP addresses and port numbers of the connection.
+&*Note*&: If an expansion is used for the option,
+care should be taken that a resonable value results for
+non-SMTP messages.
+.wen
+

 .cindex "RCPT" "maximum number of incoming"
 &*Note*&: The RFCs specify that an SMTP server should accept at least 100
 RCPT commands in a single message.
 .cindex "RCPT" "maximum number of incoming"
 &*Note*&: The RFCs specify that an SMTP server should accept at least 100
 RCPT commands in a single message.


@@ -22792,8 +22834,11 @@ If unset, or expanding to an empty string, no filtering is done.
 When the message is about to be written out, the command specified by
 &%transport_filter%& is started up in a separate, parallel process, and
 the entire message, including the header lines, is passed to it on its standard
 When the message is about to be written out, the command specified by
 &%transport_filter%& is started up in a separate, parallel process, and
 the entire message, including the header lines, is passed to it on its standard

-input (this in fact is done from a third process, to avoid deadlock). The
-command must be specified as an absolute path.
+input (this in fact is done from a third process, to avoid deadlock).
+The command must be specified as an absolute path.
+
+The process run by the command must use its standard input as the message
+data to be transformed, and write the results on its standard output.

 
 The lines of the message that are written to the transport filter are
 terminated by newline (&"\n"&). The message is passed to the filter before any
 
 The lines of the message that are written to the transport filter are
 terminated by newline (&"\n"&). The message is passed to the filter before any


@@ -24828,7 +24873,7 @@ Exim, and each argument is separately expanded, as described in section
 No part of the resulting command may be tainted.
 
 
 No part of the resulting command may be tainted.
 
 

-.option environment pipe string&!! unset
+.option environment pipe "string list&!!" unset

 .cindex "&(pipe)& transport" "environment for command"
 .cindex "environment" "&(pipe)& transport"
 This option is used to add additional variables to the environment in which the
 .cindex "&(pipe)& transport" "environment for command"
 .cindex "environment" "&(pipe)& transport"
 This option is used to add additional variables to the environment in which the


@@ -25334,6 +25379,13 @@ over a single TCP/IP connection. If the value is zero, there is no limit.
 For testing purposes, this value can be overridden by the &%-oB%& command line
 option.
 
 For testing purposes, this value can be overridden by the &%-oB%& command line
 option.
 

+.new
+.cindex "ESMTP extensions" LIMITS
+If the peer advertises a LIMITS extension with a MAILMAX value,
+and either TLSS is in use or was not advertised,
+that value also constrains the result of this option.
+.wen
+

 
 .option dane_require_tls_ciphers smtp string&!! unset
 .cindex "TLS" "requiring specific ciphers for DANE"
 
 .option dane_require_tls_ciphers smtp string&!! unset
 .cindex "TLS" "requiring specific ciphers for DANE"


@@ -25546,15 +25598,24 @@ load-balancer, matching the session stored in the client's cache.
 
 Exim can pull out a server name, if there is one, from the response to the
 client's SMTP EHLO command.
 
 Exim can pull out a server name, if there is one, from the response to the
 client's SMTP EHLO command.

-The default value of this option:
+For normal STARTTLS use, the default value of this option:

 .code
     ${if and { {match {$host} {.outlook.com\$}} \
                {match {$item} {\N^250-([\w.]+)\s\N}} \
             } {$1}}
 .endd
 suffices for one known case.
 .code
     ${if and { {match {$host} {.outlook.com\$}} \
                {match {$item} {\N^250-([\w.]+)\s\N}} \
             } {$1}}
 .endd
 suffices for one known case.

+

 During the expansion of this option the &$item$& variable will have the
 server's EHLO response.
 During the expansion of this option the &$item$& variable will have the
 server's EHLO response.

+
+.new
+For TLS-on-connect connections we do not have an EHLO
+response to use. Because of this the default value of this option is
+set to a static string for those cases, meaning that resumption will
+always be attempted if permitted by the &%tls_resumption_hosts%& option.
+.wen
+

 The result of the option expansion is included in the key used to store and
 retrieve the TLS session, for session resumption.
 
 The result of the option expansion is included in the key used to store and
 retrieve the TLS session, for session resumption.
 


@@ -25901,11 +25962,20 @@ each set of addresses is treated independently, and
 so can cause parallel connections to the same host if &%remote_max_parallel%&
 permits this.
 
 so can cause parallel connections to the same host if &%remote_max_parallel%&
 permits this.
 

+.new
+.cindex "ESMTP extensions" LIMITS
+If the peer advertises a LIMITS extension with a RCPTMAX value,
+and either TLSS is in use or was not advertised,
+that value also constrains the result of this option
+and no parallel connections will be caused on meeting the RCPTMAX limit.
+.wen
+

 
 .option message_linelength_limit smtp integer 998
 .cindex "line length" limit
 This option sets the maximum line length, in bytes, that the transport
 will send.  Any messages with lines exceeding the given value
 
 .option message_linelength_limit smtp integer 998
 .cindex "line length" limit
 This option sets the maximum line length, in bytes, that the transport
 will send.  Any messages with lines exceeding the given value

+(before a transport filter, if any)

 will fail and a failure-DSN ("bounce") message will if possible be returned
 to the sender.
 The default value is that defined by the SMTP standards.
 will fail and a failure-DSN ("bounce") message will if possible be returned
 to the sender.
 The default value is that defined by the SMTP standards.


@@ -25931,6 +26001,14 @@ If the connection is DANE-enabled then this option is ignored;
 only messages having the domain used for the DANE TLSA lookup are
 sent on the connection.
 
 only messages having the domain used for the DANE TLSA lookup are
 sent on the connection.
 

+.new
+.cindex "ESMTP extensions" LIMITS
+If the peer advertises a LIMITS extension with a RCPTDOMAINMAX value,
+and either TLSS is in use or was not advertised,
+this option is regarded as being false.
+.wen
+
+

 .option port smtp string&!! "see below"
 .cindex "port" "sending TCP/IP"
 .cindex "TCP/IP" "setting outgoing port"
 .option port smtp string&!! "see below"
 .cindex "port" "sending TCP/IP"
 .cindex "TCP/IP" "setting outgoing port"


@@ -30687,6 +30765,10 @@ and the &%acl_smtp_mime%& ACLs.
 The &%acl_smtp_dkim%& ACL is available only when Exim is compiled with DKIM support
 enabled (which is the default).
 
 The &%acl_smtp_dkim%& ACL is available only when Exim is compiled with DKIM support
 enabled (which is the default).
 

+If, for a specific message, an ACL control
+&*dkim_disable_verify*&
+has been set, this &%acl_smtp_dkim%& ACL is not called.
+

 The ACL test specified by &%acl_smtp_dkim%& happens after a message has been
 received, and is executed for each DKIM signature found in a message.  If not
 otherwise specified, the default action is to accept.
 The ACL test specified by &%acl_smtp_dkim%& happens after a message has been
 received, and is executed for each DKIM signature found in a message.  If not
 otherwise specified, the default action is to accept.


@@ -35323,9 +35405,10 @@ The arguments are as follows:
 (the -D file). The file is open for reading and writing, but updating it is not
 recommended. &*Warning*&: You must &'not'& close this file descriptor.
 
 (the -D file). The file is open for reading and writing, but updating it is not
 recommended. &*Warning*&: You must &'not'& close this file descriptor.
 

-The descriptor is positioned at character 19 of the file, which is the first
-character of the body itself, because the first 19 characters are the message
-id followed by &`-D`& and a newline. If you rewind the file, you should use the
+The descriptor is positioned at character 26 of the file, which is the first
+character of the body itself, because the first 26 characters (19 characters
+before Exim  4.97) are the message id followed by &`-D`& and a newline.
+If you rewind the file, you should use the

 macro SPOOL_DATA_START_OFFSET to reset to the start of the data, just in
 case this changes in some future version.
 .next
 macro SPOOL_DATA_START_OFFSET to reset to the start of the data, just in
 case this changes in some future version.
 .next


@@ -36441,8 +36524,6 @@ other MTAs, the way Exim handles line endings for all messages is now as
 follows:
 
 .ilist
 follows:
 
 .ilist

-LF not preceded by CR is treated as a line ending.
-.next

 CR is treated as a line ending; if it is immediately followed by LF, the LF
 is ignored.
 .next
 CR is treated as a line ending; if it is immediately followed by LF, the LF
 is ignored.
 .next


@@ -36457,7 +36538,10 @@ people trying to play silly games.
 .next
 If the first header line received in a message ends with CRLF, a subsequent
 bare LF in a header line is treated in the same way as a bare CR in a header
 .next
 If the first header line received in a message ends with CRLF, a subsequent
 bare LF in a header line is treated in the same way as a bare CR in a header

-line.
+line and a bare LF in a body line is replaced with a space.
+.next
+If the first header line received in a message does not end with CRLF, a subsequent
+LF not preceded by CR is treated as a line ending.

 .endlist
 
 
 .endlist
 
 


@@ -39040,7 +39124,7 @@ selection marked by asterisks:
 .irow &`deliver_time`&                 &nbsp; "time taken to attempt delivery"
 .irow &`delivery_size`&                        &nbsp; "add &`S=`&&'nnn'& to => lines"
 .irow &`dkim`&                         *       "DKIM verified domain on <= lines"
 .irow &`deliver_time`&                 &nbsp; "time taken to attempt delivery"
 .irow &`delivery_size`&                        &nbsp; "add &`S=`&&'nnn'& to => lines"
 .irow &`dkim`&                         *       "DKIM verified domain on <= lines"

-.irow &`dkim_verbose`&                 &nbsp; "separate full DKIM verification result line, per signature"
+.irow &`dkim_verbose`&                 &nbsp; "separate full DKIM verification result line, per signature; DKIM signing"

 .irow &`dnslist_defer`&                        *       "defers of DNS list (aka RBL) lookups"
 .irow &`dnssec`&                       &nbsp; "DNSSEC secured lookups"
 .irow &`etrn`&                         *       "ETRN commands"
 .irow &`dnslist_defer`&                        *       "defers of DNS list (aka RBL) lookups"
 .irow &`dnssec`&                       &nbsp; "DNSSEC secured lookups"
 .irow &`etrn`&                         *       "ETRN commands"


@@ -39084,7 +39168,7 @@ selection marked by asterisks:
 .irow &`tls_peerdn`&                   &nbsp; "TLS peer DN on <= and => lines"
 .irow &`tls_resumption`&               &nbsp; "append * to cipher field"
 .irow &`tls_sni`&                      &nbsp; "TLS SNI on <= lines"
 .irow &`tls_peerdn`&                   &nbsp; "TLS peer DN on <= and => lines"
 .irow &`tls_resumption`&               &nbsp; "append * to cipher field"
 .irow &`tls_sni`&                      &nbsp; "TLS SNI on <= lines"

-.irow &`unknown_in_list`&              &nbsp; "DNS lookup failed in list match"
+.irow &`unknown_in_list`&              &nbsp; "lookup failed in list match"

 .irow &`all`&                          &nbsp; "&*all of the above*&"
 .endtable
 See also the &%slow_lookup_log%& main configuration option,
 .irow &`all`&                          &nbsp; "&*all of the above*&"
 .endtable
 See also the &%slow_lookup_log%& main configuration option,


@@ -39166,6 +39250,10 @@ verifies successfully a tag of DKIM is added, with one of the verified domains.
 .cindex log "DKIM verification"
 .cindex DKIM "verification logging"
 &%dkim_verbose%&: A log entry is written for each attempted DKIM verification.
 .cindex log "DKIM verification"
 .cindex DKIM "verification logging"
 &%dkim_verbose%&: A log entry is written for each attempted DKIM verification.

+.new
+Also, on message delivery lines signing information (domain and selector)
+is added, tagged with DKIM=.
+.wen

 .next
 .cindex "log" "dnslist defer"
 .cindex "DNS list" "logging defer"
 .next
 .cindex "log" "dnslist defer"
 .cindex "DNS list" "logging defer"


@@ -39503,7 +39591,8 @@ added to the log line, preceded by SNI=.
 .next
 .cindex "log" "DNS failure in list"
 &%unknown_in_list%&: This setting causes a log entry to be written when the
 .next
 .cindex "log" "DNS failure in list"
 &%unknown_in_list%&: This setting causes a log entry to be written when the

-result of a list match is failure because a DNS lookup failed.
+result of a list match is failure because a DNS lookup failed, or because
+a bad IP address was in the list.

 .endlist
 
 
 .endlist
 
 


@@ -41511,8 +41600,11 @@ Exim's DKIM implementation allows for
 .olist
 Signing outgoing messages: This function is implemented in the SMTP transport.
 It can co-exist with all other Exim features
 .olist
 Signing outgoing messages: This function is implemented in the SMTP transport.
 It can co-exist with all other Exim features

-(including transport filters)
-except cutthrough delivery.
+(including transport filters) except cutthrough delivery.
+.new
+However, signing options may not depend on headers modified by
+routers, the transport or a transport filter.
+.wen

 .next
 Verifying signatures in incoming messages: This is implemented by an additional
 ACL (acl_smtp_dkim), which can be called several times per message, with
 .next
 Verifying signatures in incoming messages: This is implemented by an additional
 ACL (acl_smtp_dkim), which can be called several times per message, with


@@ -41698,7 +41790,7 @@ variables here.
 .option dkim_sign_headers smtp string&!! "see below"
 If set, this option must expand to a colon-separated
 list of header names.
 .option dkim_sign_headers smtp string&!! "see below"
 If set, this option must expand to a colon-separated
 list of header names.

-Headers with these names, or the absence or such a header, will be included
+Headers with these names, or the absence of such a header, will be included

 in the message signature.
 When unspecified, the header names listed in RFC4871 will be used,
 whether or not each header is present in the message.
 in the message signature.
 When unspecified, the header names listed in RFC4871 will be used,
 whether or not each header is present in the message.


@@ -41720,10 +41812,11 @@ name will be appended.
 .option dkim_timestamps smtp integer&!! unset
 This option controls the inclusion of timestamp information in the signature.
 If not set, no such information will be included.
 .option dkim_timestamps smtp integer&!! unset
 This option controls the inclusion of timestamp information in the signature.
 If not set, no such information will be included.

-Otherwise, must be an unsigned number giving an offset in seconds from the current time
-for the expiry tag
-(eg. 1209600 for two weeks);
-both creation (t=) and expiry (x=) tags will be included.
+.new
+Otherwise, must be an unsigned number giving an offset in seconds from the
+current time for the expiry tag (e.g. 1209600 for two weeks); both creation
+(t=) and expiry (x=) tags will be included unless the offset is 0 (no expiry).
+.wen

 
 RFC 6376 lists these tags as RECOMMENDED.
 
 
 RFC 6376 lists these tags as RECOMMENDED.
 


@@ -42221,10 +42314,16 @@ The lookup will return the same result strings as can appear in
 
 .subsection "SRS (Sender Rewriting Scheme)" SECTSRS
 .cindex SRS "sender rewriting scheme"
 
 .subsection "SRS (Sender Rewriting Scheme)" SECTSRS
 .cindex SRS "sender rewriting scheme"

+.cindex VERP "variable envelope return path"

 
 SRS can be used to modify sender addresses when forwarding so that
 SPF verification does not object to them.
 
 SRS can be used to modify sender addresses when forwarding so that
 SPF verification does not object to them.

-It operates by encoding the original envelope sender in a new
+It can also be used to identify a received bounce message as
+likely (or not) having been trigged by a message from the
+local system, and for identifying dead addresses in mailing lists.
+It is one implementation of a VERP (Variable Envelope Return Path) method.
+
+SRS operates by encoding the original envelope sender in a new

 sender local part and using a domain run by the forwarding site
 as the new domain for the sender.  Any DSN message should be returned
 to this new sender at the forwarding site, which can extract the
 sender local part and using a domain run by the forwarding site
 as the new domain for the sender.  Any DSN message should be returned
 to this new sender at the forwarding site, which can extract the