My exim4 server rejects a lot of spam emails due to failed sender verification. However, those result in lots of frozen messages, which in turn spam my postmaster inbox. Example line from exim log:
2020-08-13 15:39:28 [25544] H=(mail.foo,com) [::1]:14347 I=[::1]:25 F=<[email protected]> rejected RCPT <foobar@localhost>: Sender verify failed
And then I will get a notification like this in my postmaster account:
Message 1k69l3-0007T7-Hv has been frozen (delivery error message).
The sender is <>.
The following address(es) have yet to be delivered:
[email protected]: Unrouteable address
I guess my exim4 wanted to send a bounce message, which in turn can't be delivered. How can I avoid this?
Turns out this wasn't really a problem of my exim. The user's fetchmail was set to deliver bouncemails. However that bouncemail was supposed to go to an undeliverable address. So not much exim could do about this... The loop occured because fetchmail was set to softbounce, which retries the mail over and over again, instead of deleting the mail upon a hard error.