RFC 2822 permits (as did 822) a quoted string which
consists solely of a pair of DQUOTE characters. It's
not clear what the semantics of that are or ought to be.
it doesn't have to have semantics, since quoted-strings
can be used in places where they are opaque. if someone wants their
email address to be ""@example.com this is perfectly legal.
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.
Keith