outgoing mail quirk (error 5550)

[Rhea Hirshman]

This morning I when I went to send a couple of saved messages, I received
the following error alert:

Error: A message in your Outbox could not be sent using the account "RRH."
The server does not recognize the recipients or the server refuses to let
you send mail.
Explanation: Mail error. Unknown address.
Error 5550.

But what doesn't make sense is that (1) A couple of other messages went out
with no problem at the same time. (2) The addresses in the problem message
are addresses I use all the time ‹ used as recently as a day or two ago. So
why all of a sudden am I getting an error message about addresses?

And if one of the addresses is corrupt, why doesn't the message go through
and kick back to me the one with the dysfunctional address? That does happen
sometimes, allowing the message to get through to most of the recipients.
And is there some way to identify the "bad" address so I can delete it and
send the message to others? Or is this not really about a bad address but
some other weird Entourage quirk?

Entourage 2001 on a Mac G4 with system 9.2.2.

Thanks.

‹ Rhea