+2. There is a new acl, set by acl_not_smtp_start, which is run right at the
+ start of receiving a non-SMTP message, before any of the message has been
+ read. (This is the analogue of the acl_smtp_predata ACL for SMTP input.) The
+ result of this ACL is ignored; it cannot be used to reject a message. If
+ you really need to, you could set a value in an ACL variable here and reject
+ based on that in the acl_not_smtp ACL. However, this ACL can be used to set
+ controls, and in particular, it can be used to set control=suppress_local_
+ fixups, which cannot be used in the acl_not_smtp ACL because by the time
+ that ACL is run, it is too late. When the acl_not_smtp_start ACL is run, the
+ sender and recipients are known, so the "senders" and "sender_domains"
+ conditions and $sender_address and $recipients variables can be used.
+ Variables such as $authenticated_ sender are also available. It is possible
+ to specify added header lines in this ACL.
+