Caused by some idiot who does not know how to setup their mail server
correctly, looks like a MicroSloth product as well. If you have
control of your mail server, tell /etc/hosts.allow to allow all SMTP
except gateway.dr.dk (129.142.20.129), let their server fill up with
undeliverable autoreplies. So far I have refused 31 connections from
there.
Following up my own mail, add irvine.ast.com (165.164.128.2) to the
exclusion list. That is a cc:Maul gateway gone wrong.