At 10:05 +0300 on 06/16/2007, Kari Hurtta wrote about
draft-klensin-rfc2821bis-04: VRFY and EXPN syntax:
--- start quote ------
"User name" is a fuzzy term and has been used deliberately. An
implementation of the VRFY or EXPN commands MUST include at least
recognition of local mailboxes as "user names". However, since
current Internet practice often results in a single host handling
mail for multiple domains, hosts, especially hosts that provide this
functionality, SHOULD accept the "local-part(_at_)domain" form as a "user
name"; hosts MAY also choose to recognize other strings as "user
names".
--- end quote ------
However later actual syntax seems not allow "@domain" which is "SHOULD"
on here.
When a SMTP Server hosts multiple domains (at least when Virtual
Hosting), it knows (or should know via the FQDN passed when the
remote client connects) which domain it is acting as. Thus a
Local-Part only should be adequate. A note in the quote above of the
requirement that it knows who it is acting as might be helpful to
clarify the situation.