Are you sure it is trying to send it to the no-longer-available newsgroup?
When you do the followup yarn gives you a message to edit that looks just
like a real newsgroup followup--with a Newsgroups: header and without a
To: header. But if you look at your replies, you'll see it in the mail
folder, with the correct To: header.
But if it really is trying to post your followups as news, perhaps you
could show us the relevant line from your active file? I just tried
switching one of my gatewayed newsgroup/mailing-lists to post to the
list instead of the newsgroup and it worked just fine. The correct way
to do this is with a newgroup command:
newgroup group.name.here keepdays maxkeepdays email@address.here
(e.g. newgroup comp.sys.hp.mpe 30 90 HP3000-L@RAVEN.UTC.EDU)
There is absolutely no reason to directly edit the active file...a
newgroup command will modify the settings of an existing group.
If you are running yarn at the time you do the newgroup command, you
will need to exit and restart yarn to have the change take effect.
If all else fails, show us your active file and the headers from the
incorrect followup and maybe someone will have some ideas for you.