among them are: anonymous(_at_)[0(_dot_)0(_dot_)0(_dot_)0], anonymous(_at_)[127(_dot_)0(_dot_)0(_dot_)1],
anonymous(_at_)[127(_dot_)255(_dot_)255(_dot_)255], anonymous(_at_)[::0], and anonymous(_at_)[::1].
(Don't these have mostly the same DNS problems as blah(_at_)blah(_dot_)invalid?)
they shouldn't. any MTA that tries to do a DNS lookup of a domain
literal is seriously broken.
(and at least at one time there was a popular MTA that would look up
the PTR record for the IP address, get a domain name, and try to
substitute that domain name - which is completely ridiculous. the
whole point of using a domain literal is to connect directly to the
SMTP server at that address.)