Domain get's added to blacklist again and again

We have got a number of pc’s with emclient. With 1 of them we have got the problem that our own domain get’s added to the blacklist. Ofcourse we have removed this, but it still get’s added again.

I’ve also tried to remove the blacklist rule, but after a few hours this reappears again.

So how can I prevent this from happening on this specific installation.
version: 7.2.38732.0