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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=<qsdqsdqsdqsdsqdsqdqsdqsdsqdqsd@phativerthe.com> 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:
qsdqsdqsdqsdsqdsqdqsdqsdsqdqsd@phativerthe.com: Unrouteable address

I guess my exim4 wanted to send a bounce message, which in turn can't be delivered. How can I avoid this?

Arne
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  • I'm not familiar with exim (it's a rather niche email server these days) but your mail server shouldn't be accepting these messages to begin with. – Michael Hampton Aug 13 '20 at 14:30
  • It is not accepting them. It tries to do a sender verification, which fails. And I guess the proper way is to notify the sender, which fails again. I think I might need to break that chain right there. Not sure if that violates SMTP etiquette, but I guess the Spammers don't care... – Arne Aug 13 '20 at 14:34
  • I think this spam mail produced a loop locally for some reason. The ipv6 address hints at that. I deleted all pending mails from this address from my input queue. Let’s see if that helped. Question is how this loop happened? – Arne Aug 13 '20 at 16:58
  • Nope. That wasn’t it. I am still receiving those mails. Hm. – Arne Aug 13 '20 at 17:46
  • Found it. One of my users is using fetchmail to collect emails from another server. That’s where this email is coming from. And that’s why I get the error every 5 minutes. That’s his polling interval. – Arne Aug 13 '20 at 17:56

1 Answers1

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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.

Arne
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