-Early pipelining support
-------------------------
-Ref: https://datatracker.ietf.org/doc/draft-harris-early-pipe/
-
-If compiled with EXPERIMENTAL_PIPE_CONNECT support is included for this feature.
-The server advertises the feature in its EHLO response, currently using the name
-"X_PIPE_CONNECT" (this will change, some time in the future).
-A client may cache this information, along with the rest of the EHLO response,
-and use it for later connections. Those later ones can send esmtp commands before
-a banner is received.
-
-Up to 1.5 roundtrip times can be taken out of cleartext connections, 2.5 on
-STARTTLS connections.
-
-In combination with the traditional PIPELINING feature the following example
-sequences are possible (among others):
-
-(client) (server)
-
-EHLO,MAIL,RCPT,DATA ->
- <- banner,EHLO-resp,MAIL-ack,RCPT-ack,DATA-goahead
-message-data ->
-------
-
-EHLO,MAIL,RCPT,BDAT ->
- <- banner,EHLO-resp,MAIL-ack,RCPT-ack
-message-data ->
-------
-
-EHLO,STARTTLS ->
- <- banner,EHLO-resp,TLS-goahead
-TLS1.2-client-hello ->
- <- TLS-server-hello,cert,hello-done
-client-Kex,change-cipher,finished ->
- <- change-cipher,finished
-EHLO,MAIL,RCPT,DATA ->
- <- EHLO-resp,MAIL-ack,RCPT-ack,DATA-goahead
-
-------
-(tls-on-connect)
-TLS1.2-client-hello ->
- <- TLS-server-hello,cert,hello-done
-client-Kex,change-cipher,finished ->
- <- change-cipher,finshed
- <- banner
-EHLO,MAIL,RCPT,DATA ->
- <- EHLO-resp,MAIL-ack,RCPT-ack,DATA-goahead
-
-Where the initial client packet is SMTP, it can combine with the TCP Fast Open
-feature and be sent in the TCP SYN.
-
-
-A main-section option "pipelining_connect_advertise_hosts" (default: *)
-and an smtp transport option "hosts_pipe_connect" (default: unset)
-control the feature.
-
-If the "pipelining" log_selector is enabled, the "L" field in server <=
-log lines has a period appended if the feature was advertised but not used;
-or has an asterisk appended if the feature was used. In client => lines
-the "L" field has an asterisk appended if the feature was used.
-
-The "retry_data_expire" option controls cache invalidation.
-Entries are also rewritten (or cleared) if the adverised features
-change.
-
-
-NOTE: since the EHLO command must be constructed before the connection is
-made it cannot depend on the interface IP address that will be used.
-The string "$sending_ip_address" is checked for; if it appears in helo_data
-and "def:sending_ip_address" does not, the facility is disabled.
-
-Transport configurations should be checked for this. An example avoidance:
-
- helo_data = ${if def:sending_ip_address \
- {${lookup dnsdb{>! ptr=$sending_ip_address} \
- {${sg{$value} {^([^!]*).*\$} {\$1}}} fail}} \
- {$primary_hostname}}
-
-
-
-
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