Thank you Victor, your detailed analysis of the TCP dump made me look into the 
right direction.

Virtualisation (KVM) is also used here, but that was not the reason for the 
behaviour. It also wasnt TCP offloading, although used.

The culprit was a firewall. I am a bit limited as to what I can expose of that 
customers infrastructure, though. I disconnected all the security boxes and saw 
incoming mails from the servers without problems. Then I 
reconnected boxes one by one and found the troublemaker.

Hope this will be of help to somebody, and thanks again to Victor for the 
helpful analysis.

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