Hi there,
just want to let you know what we and our partner independently found out:
Remember this if you troubleshoot problems concerning sending mails to just a few special domains.
Imagine a mis-configured mailserver on the net - and there are some - that just drop a connection when one of the receiving criteria is not matched. In our cases we saw the following: if a mail is larger than the receiving-smtp limit of e.g. 10MB the receiving server drops the connection - without any smtp response. (bad behavior of the receiving server)
Here is what Astaro does: resend the 10MB Mail for 24 hours (!) a 100 times creating some GB mailtraffic - before it sends a first "message delayed" information to the user. This goes on until day 3 - when the message will be bounced by the Astaro.
As if this wasn't enough of bad mail handling another important thing happens:
the Astaro is working down the mailqueue from top down - trying to resend the problematic mail first (as we know: unsuccessfully) and caches this mailserver as unreachable - resulting in OTHER (small / receiving-criteria-matched) mails to the same domain in NEVER BEING TRIED TO SEND - the mailqueue grows...
As you can imagine this is disastrous for messaging: users are sending their mails a whole day long and this *one* message causes all other messages not being delivered. So after 24 hours the "bad-mail" sender receives the "message delayed" message - and if no one cares about the mailtransport after this informational mail, there is a delay of three days until the first messages to the *working* receiver-SMTP-Server are getting *started* being delivered.
This could be called a partial DOS - unintentionally and very easy to "exploit" from internal side...
Due to this is not a bug regarding to Astaro support this had to be delivered as a feature request.
In the feature-request forum I suggested to implement a "maximum-transmission-attempts" counter that bounces the mail if sending has *successfully* begun and has been aborted more then ... lets say 10 times.
Hope you will never stumble upon this,
Best regards,
Thomas
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