What is the build date on LISTSERV? (Do a "show license")

The reason that message was rejected was that there was a
multipart/alternative within a multipart/mixed, and
multipart/alternative is not among the allowed attachment types.

Early versions of the Attachment= keyword only looked at one level of
MIME. More recent versions of LISTSERV process nested MIME messages.
You may find the behavior in more recent versions of LISTSERV more to
your liking.

Francoise