+A number of ideas arose following a discussion on the mailing list. I record
+them here so that they don't get lost. The motivations were to support the 551
+bounce code and enhanced status codes. Suggestions are to add a new ACL
+feature, possibly one of:
+
+ errorcode = 511
+ control = errorcode=551
+ message = 551 xxxx
+
+where in the last case, it's recognized by being 3 digits. In all cases, the
+first digit must be "right" for the circumstance - ignore or fault if not?
+
+To handle ESC, perhaps a new variable called $smtp_errorcode, settable by an
+option in a router when it fails, would do the trick. It could be used in any
+of the above modifiers.
+------------------------------------------------------------------------------
+
+(355) 30-Jun-06 ? Facility to permit experiments with SMTP extensions
+
+This is what was suggested:
+
+- adding some expansion variables: $ehlo_extensions (which will
+ hold the remote server supported smtp extensions announced
+ in the ehlo) and $rcpt_arguments with any RCPT extra argument
+
+- a main configuration option for adding ehlo extensions to the
+ ehlo response, like:
+
+ extra_ehlo_extensions = XFOO : XBAR
+
+- a extra option for the smtp transport to add arguments to
+ the RCPT TO command, like:
+
+ rcpt_args = FOO=BAR (will make exim issue RCPT TO:<a@b.c> FOO=BAR
+ when delivering that message)
+
+- a new acl for unknown smtp commands
+
+This should be very simple to implement and will allow to make
+some experiments and implement custom extensions, i.e. one to
+known if remote client will redirect on 551 or not. Also the acl
+for unknown smpt command could be used for other purposes, like
+to dectect and react to some kiddies that send things like
+http://... on the smtp port.