I don't know how to start a news thread. I can _reply_ to a post, and I get a composer window with a Post-To: header filled in appropriately. But if I try to create a new message, even if I select my 'news' account as the source, I can't enable the Post-To: header in the composer. It remains greyed-out.
Right, the composer needs to be smarter about managing its header fields. They currently get enabled/disabled permanently when the composer window is created instead of reacting to which type of account you've chosen. So unless you're replying to a newsgroup post (or using Post New Message to Folder), it assumes you want the mail message set, which excludes Post-To. Preferences for visible headers is screwy too, because we only remember one set of header preferences (as opposed to a set for mail messages and another set for newsgroup posts). Hope to get this straightened out for the next release.
I have a patch to finally fix this for Fedora 12. Moving this to an equivalent upstream bug. See [1] for further updates. [1] http://bugzilla.gnome.org/show_bug.cgi?id=274117