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I just came across this problem myself.

What I'm trying to do is use an auditor to extract special
information from the message content of incoming mail, then
remove the extracted information from the contents. If contents
are now empty, I'd like to veto the creation of the message, but
continue with the rest of the operation (e.g. create or
modify an
issue, but without a new message).

To be able to do that we'd need two kinds of Reject exceptions:

- one that aborts an operation as a whole (abort creating a new
  issue do to a badly formatted message), i.e. ValueError

- and one that only vetos the current operation, let's call it
  SingleReject.

While a ValueError would be allowed to traverse up through the
call hierarch to the general error handling stuff, the
latter would
have to be caught at various places, like in mailgw.py line 885:

    message_id = self.db.msg.create(author=author,...

where a SingleReject should be treated similar to a message
with empty content.

I don't know enough about roundup's internals yet to understand
what else might be affected. Certainly client.py though.

Am I making any sense? ;-)