Puts you into the MOO mail system to register a complaint (or, conceivably, a compliment) with the wizards. The rest of the command line (the <anything> ... part) is used as the subject line for the message. More information on using the MOO mail system is given once you're in it.
You may hear back from the wizards eventually; see "help @mail" for how to read their reply.
Example:
Munchkin types:
@gripe The little bird
"How come I can't ever see the little bird in the cuckoo clock?
" -- A frustrated player
send
and, somewhat later, the wizards reply with a note about being sure to look while the clock is chiming.