The standard (as in most common setup) for djb's qmail, was to accept/forward mail for <prefix>@user.example.com to [email protected], as in: [email protected] rather than [email protected].

While I believe the + was common for postfix, exim and sendmail?

There even was a spam-fighting scheme that used this - it took a key, and optional date, and a sender address - and generated a unique address you could give out, that was only valid for a certain time, for a certain set of senders.

If the system received a mail with a from address that didn't match the cryptologically signed to-address ([email protected]) - the mail was held back, and the system generated a reply, with a signed reply-to address. A sort of manual grey-listing.