+3. When an SMTP error message is specified in a "message" modifier in an ACL,
+ or in a :fail: or :defer: message in a redirect router, Exim now checks the
+ start of the message for an SMTP error code. This consists of three digits
+ followed by a space, optionally followed by an extended code of the form
+ n.n.n, also followed by a space. If this is the case and the very first
+ digit is the same as the default error code, the code from the message is
+ used instead. If the very first digit is incorrect, a panic error is logged,
+ and the default code is used. This is an incompatible change, but it is not
+ expected to affect many (if any) configurations. It is possible to suppress
+ the use of the supplied code in a redirect router by setting the
+ smtp_error_code option false. In this case, any SMTP code is quietly
+ ignored.
+