I have setup a catch-all mailbox (mail@domain.com). If i send a mail to someone@domain.com, this should ideally be routed to mail@domain.com. The problem is, that it only works from domains sending mail on the same server.

If i send a mail to someone@domain.com from a domain sitting on another server, it returns an error "Unknown user"

Please can someone advise if I am doing something wrong, or is this a common problem.