But why would anyone need it?
I map aaa(_at_)bbb(_dot_)iecc(_dot_)com to bbb-aaa(_at_)iecc(_dot_)com for most
values of bbb. It
gives each mail user his own domain and makes it less obvious that
tagged addresses are tagged addresses.
Ny system is small enough that it would be no big deal to put in
explicit records for everyone, but I believe there are ISPs who do
this and have enough users that the only reasonable implementation
is with a wildcard.
There's also a variety of gateway applications where I find it useful
to put some of the gateway info as a subdomain. It's a useful
technique, and it would be unfortunate to break it.
Regards,
John Levine, johnl(_at_)taugh(_dot_)com, Taughannock Networks, Trumansburg NY
http://www.taugh.com