Thunderbird is censoring-blocking x-posting to certain groups

It's not a provider issue because the exact same message can be sent 
to the same groups using the same provider and different software such as 
Agent or Xnews.

The issue appears with certain sport and political groups.

The groups are resident on the server, so it not's the usual stupid 
Thunderbird issue of being unable to reply to messages with a group that 
is  not on the posting server.  That's really stupid anyway.  There are 
many servers that don't have common groups.

Be aware that if you use Thunderbird, your content will be censored.