My general advice: do not trust ONLY this message. The real error is elsewhere. For instance, I wanted to post to proxas.free.adsl, and until wireshark gave the me the answer, I lost 2 hours searching on the net...
Trust me: wireshark the communication.
For me the answer was:
POST
340 340 <4ea5d155$0$3203$426a74cc@news.free.fr> (desired)
Content-Type: text/plain; charset="UTF-8"
From: etc...
441 441 437 UTF-8 charset is not allowed in group proxad.free.adsl.degroupage
Yes! Just because I was in UTF-8 charset...