[Mimedefang] Undesired consequence of stream_by_recipient

[Jim Hatfield]

I didn't appreciate that stream_by_recipient causes mail
to be accepted before the validity of the recipient addresses
has been checked. It seems that if any of the recipient
addresses are invalid, instead of errors being generated at
the original "rcpt to" phase, they don't show up until the 
queued mails are processed, whereupon bounce messages are
generated.

jim