Dave:

This makes perfect sense.  If you list is SEND=PRIVATE, and the user has
multiple e-mail addresses by which they can receive e-mail, but only
generate mail from a particular (non-subscribed) address, then this is the
result.

E.g., today you can e-mail me as [log in to unmask], [log in to unmask],
[log in to unmask], [log in to unmask], and [log in to unmask]  If I were
subscribed to your list as [log in to unmask], and received list distributions
perfectly fine, yet I generate mail as [log in to unmask], then my mail TO
the list would bounce.

You need to provide more precise details: subscriber userid abstracted from
your list, bounced mail (or direct mail from the poster).  Also interrogate
your list's FILTER= to see if any specific or generic addresses.  Likewise,
issue a LISTSERV command:

SERVE userid@host

just to be sure that they weren't SERVE OFF due to too many invalid commands.

/Pete

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