Unable to send emails to main mailbox.

Unable to send emails to main mailbox.

I have 4 mailboxes set up at the minute but the main email that is super administrator ( admin@mydomain.co.uk) is unable to receive emails.  Everytime I try to send a test email to admin@mydomain.co.uk, I get an undelivered notification saying user unknown.  I have tried everything including aliases, groups and have settled on having each email address I need as a separate user and they all work except primary admin email.  Any ideas?