[exim] Bad destination mailbox address question

Some help from you experts please as searches of the Exim archives or
the Wiki did not turn up anything useful.


As a part-time postmaster of a small mail host setup which runs Exim
4.63 I get some reports on spam that failed because of "Bad destination
mailbox address".


Where is this generated by Exim? Is it by a router or does it happen
earlier?


Why I am asking is that most of this spam is addressed to
'Mailer-Daemon@???' <mailto:'Mailer-Daemon@local.domain'> .

In an effort to stop this particular type of spam I added the following
to the acl_smtp_rcpt ACL:

discard hosts     = +company_relays
        condition = ${if match {${lc:$local_part}} {(mailer-daemon)}}
        logwrite  = External e-mail from '$sender_address' for
'$local_part@???' <mailto:'$local_part@local.domain'>  -
discard!

This discards spam arriving from outside the company addressed to
'mailer-daemon@???' but doesn't catch the spam that results in
the "Bad destination mailbox address" e-mail being sent to postmaster.

These type of e-mails also have 'Return-path: <>' but my acl_smtp_rcpt
ACL code doesn't catch that either.


Any ideas of how I can discard this type of spam without an e-mail being
sent to postmaster because of "Bad destination mailbox address"?


Thanks


Paul McIlfatrick