I can't think of any logical, legitimate, reason why anyone would want
to post to 'ietf-822(_at_)imc(_dot_)org' unless they were a member of the
list -
that'd be like shouting across a room and interrupting someone else's
conversation, then running away.
on the contrary, it's quite a reasonable place to ask questions about
internet mail and MIME. I wouldn't be surprised if this list only
accepted posts from subscribers - these days it's not unusual for lists
to get hundreds of spams in a day and it's simply too much trouble to
filter them - but it's not _inherently_ unreasonable for a discussion
list to accept outside traffic.
Keith