If only it were that simple. The KB article refers to a situation in which mail from the (sub)domain isn't send, but my trouble only has to do with mail forwarding.
Luckily I restarted it, but unfortunately it didn't help.
Looking at the output of
postqueue -p
I suspect it has to do with the transip.nl mailservice, because it appears the header are rewritten by the srs
0F71510E53E1 2790 Sun Oct 29 16:28:03...