[m4lvin]

This!!

I reported the same problem to posteo.de in March 2016 and still have not received a satisfactory answer, though it seems they have some counter-measures in their webmailer nowadays. The fun part was that as a "no logs" privacy-oriented provider, they were not even able to track who sent them a complaint from their own support address ¯\\_(ツ)_/¯

As a comparison: at disroot.org I found the same problem, and it took them a few hours to repair their postfix configs.