I'm looking for a way to automatically delete e-mails that are rejected by sendmail with a return code 451: Domain of sender address does not exist.

fetchmail no longer does this automatically: fetchmail FAQ

Cheers.

A 451 error could be temporary, e.g. DNS down or busy, so deletion might not be the best option.

You could try "not bouncing" the message to the "postmaster" address and sending it to /dev/null with the MTA. You might even want to deliver to a honeypot account to let SpamAssassin learn from it.

I have my setup send all bounces to a postmaster account via sendmail which then sends them on, via aliases, to me+postmaster which delivers to a "postmaster" folder in my IMAP account for manual deletion.

The problem is not what to do with the mail. It's how to delete it from the mailbox on my ISP, as fetchmail leaves it there.

Cheers.

Sorry for the confusion...

I just checked what happens on my system (fetchmail -> sendmail -> procmail) and the error reported to the logs by sendmail and fetchmail are:
"553 5.1.8 Domain of sender address user@invalid.com does not exist"
i.e. not a 451 error.
The fetchmail man page lists error code 553 as the only case where it will delete the message without delivery (or bounce).

Looks like the problem could be with the sendmail configuration. Check you have the latest software package and check that sendmail.cf is up to date, possibly built from sendmail.mc.

The error is actually generated in the "checkmail" ruleset. Search the file for the "Domain of sender address" string:

$ grep 'Domain of sender address' /etc/mail/sendmail.cf
R<TEMP> $* $#error $@ 4.1.8 $: "451 Domain of sender address " $&f " does not resolve"
R<PERM> $* $#error $@ 5.1.8 $: "553 Domain of sender address " $&f " does not exist"

The first bit is a test for temporary or permanent DNS lookup failures. So if your sendmail.cf matches mine, the problem could be with DNS lookups. Are you using any DNS blacklists?
Check your /etc/resolv.conf for non-functioning nameserver entries.

Hope this helps,

--DaveG.

Going back through my logs, I ca see instances of both 451 and 553. Now, I wonder what the difference is between the 2.

Cheers.