What would stop a mail server?


I have a windows 2003 server, on IIS6, and a plesk CP.
at first my email was workin fine. but then I installed coldfusion8 and updated my plesk CP from 8.2 to 8.3 and now I get email errors. in the plesk CP I get:

*Warning: Unable to send notification: mail() failed: mail() [<a href=‘function.mail’>function.mail</a>]: SMTP server response: 550 Requested action not taken: mailbox unavailable or not local

*and if I try to use my Horde webmail for my domain names, I get:

*There was an error sending your message: unable to add recipient [[email protected]]: Invalid response code received from server

*what would be a few things I could check into to make sure they are properly configured?