+12. The "message" modifier can now be used on acl verbs to vary the message
+ that is sent when an SMTP command is accepted. For example, in a RCPT ACL
+ you could have:
+
+ accept <some conditions>
+ message = OK, I'll allow you through today
+
+ Previously, this message modifier would have had no effect whatsoever.
+
+ IMPORTANT: The new behaviour applies to "accept" (and "discard") only if
+ there is no occurrence of "endpass" in the statement. If "endpass" is
+ present, the behaviour reverts to the old case, where "message" applies to
+ rejection. This is for backwards compatibility.
+
+ It is always possible to rewrite ACL statements so that "endpass" is not
+ needed (and indeed it is no longer used in the default configuration, and
+ is somewhat not recommended nowadays because it causes confusion.)
+
+ It is now generally true that the "message" modifier sets up a text string
+ that is expanded and used as a response message if the current statement
+ terminates the ACL. The expansion happens at the time Exim decides that the
+ ACL is to end, not at the time it processes "message". If the expansion
+ fails, or generates an empty string, the modifier is ignored.
+
+ For ACLs that are triggered by SMTP commands, the message is returned as
+ part of the SMTP response. In this situation, the message may begin with an
+ overriding SMTP response code, optionally followed by an "extended response
+ code". However, the first digit of the supplied response code must be the
+ same as would be sent by default. A panic occurs if it is not. For the
+ predata ACL, note that the default success code is 354, not 2xx.
+
+ However, notwithstanding the previous paragraph, for the QUIT ACL, unlike
+ the others, the message modifier cannot override the 221 response code.
+
+ In the case of the "connect" ACL, accepting with a message modifier
+ overrides the value of smtp_banner.
+
+ The ACL test specified by acl_smtp_helo happens when the client issues the
+ HELO or EHLO commands, after the tests specified by helo_accept_junk_hosts,
+ helo_allow_chars and helo(_try)_verify_hosts. An acceptance message
+ modifier for EHLO/HELO may not contain more than one line (it will be
+ truncated at the first newline and a panic logged), and it cannot affect
+ the EHLO options.
+
+