[Exim] acl_smtp_mailauth: how to decline correctly

Hello!

The mailauth ACL in exim 4.21 is a fine idea.
We want to use it to decide, whether the
MAIL AUTH parameter is passed or discarded.

My (working) ACL looks like this:

check_senderauth:
accept condition = ${if eq{$authenticated_sender}{$authenticated_id} {yes}{no}}
warn

I'm not quite shure about the "warn" exit. Section 23 in "doc/NewStuff"
confuses me, to be honest.

"If the ACL does not yield "accept", the value of
      $authenticated_sender is deleted."

[..]

"The acl_smtp_mailauth ACL may not return "drop" or "discard". If it defers,
    a temporary error code (451) is given for the MAIL command."

So, is the "warn" verb correct or deprecated? It seems to have the same
effect as "deny", but we'd like to make it robust for the future :)

Thank You in advance,

Marian (marian@???)