Hello, thanks for the fast replies. For me, the problem has been solved in the meantime. SORBS indeed reacted quite fast (thanks again!). What I am missing, though, is how to avoid that in the future. It is most likely to happen that from time to time someone doesn't manage how to get from the mailing lists they've subscribred to, and then sends a spam complaint, rather than contacting us.
So, we can do as much as we can on our side, but if users make errors, and miss talking to us, it will be hard to avoid it in total, so if there is any best practice on this, that would be indeed helpful. But this, as far as I understood, is off-topic, so I'll discuss offlist. :) Thanks, Florian
