I just remembered the other workaround I've seen: the MTA accepts
the mail, "splits" it according to the RCPT filtering requirements,
and then bounces those mails that are rejected by the filter(s).
As it was indicated in a different thread (about RFC2821), this is
a really bad idea.
BTW: some of those requests for "per RCPT filtering" came from
commercial vendors who use the sendmail MTA in their product, so
they most likely got those requests from their customers.
Summary: IMHO this is a fairly important feature. It will help those
MTAs which implement it to avoid the problems of the current
"workarounds". For others you can simply only accept one RCPT[*]
per transaction, especially from "unknown" sources.
PS: [*] obviously you can enhance this by accepting multiple
RCPTs if they have the same filtering requirements.