Hmm.  You are correct.  I thought the RFC's covered this case, but apparently they don't.

The email package gets used in MUA contexts, where the domain part of the address may be omitted and the MUA must fill it in before transmitting the message to the MTA.  And some MTAs will fill in the local domain if it is omitted, so actually it applies in an MTA context, too.  So we need to support this case even if it isn't covered by the RFCs.

Thanks, the revised patch looks good.