set use-invalid-address

SET USE-INVALID-ADDRESS ASK/always/never  -  whether to use strange addresses

With the usual ASK setting, MM will ask about using strange address formats 
  before trying to send to them.  MM considers as normal a format like 
  userid or userid@host.  Anything it does not recognize is considered
  strange, for example group:;@host.  

You can change it to ALWAYS (or YES), or NEVER (or NO).  Strange addresses
  probably will result in undeliverable mail that is sent back to you.

Send a message to "postmaster" for help if you have only a strange
  address to use.  It is preferable to change it to a normal format.

For more help, type "help" and one of the following:
(at any prompt:)  save-init   set   show