Likewise, a domain literal can consist solely of the
square brackets.
so? there are lots of addresses which are valid syntax but
not valid because the domain is not defined or the IP
address (in the domain literal) does not exist or is not
assigned to a host.
I'm not sure that there aren't MTAs or MUAs that would
break or pass bizarre requests to DNS when trying to
deal with [] as a domain literal.
if the M*A fails to deliver the message because of "no such host"
or some similar reason, that's fine. if the M*A fails claiming
that the address is invalid syntax, it's broken.
not that this bug is likely to make a difference in practice...
Also, in this case 2822 seems to be alone in permitting
that syntax, which is not permitted by 2821 or 1123.
well, what is valid for the message format is not necessarily
valid for SMTP - since the message format is transport-independent
while SMTP is associated with specific transports.