[Mailman-Users] Error

[rex at rexgoode.com]

The last time I posted here, my hosting company said that my newly 
upgraded plan did not support Mailman. I've been wrangling with them 
over that for a few weeks. They finally got it working, they said. All 
of my lists became active again, but now I can't send mail to them. I've 
sent this error to them, but I'm hoping someone here can help me.

When I hit send, I get:

SMTP Error (550): Failed to add recipient "listname at mydomain.com" (5.1.1 
<listname at mydomain.com>: Recipient address rejected: User unknown in 
virtual mailbox table).

This happens on all of my mailman lists. Substitute listname and 
mydomain.com appropriately, of course, but every list gets it.

Any advice I can give to my hosting company?

Rex