Have you uninstalled the programs which you suspect and then reloaded them?
Did you do a google search? Use the above "McAffee error 550" for your key words. There are a lot of folks in the same boat...
One seemed to point to a possible solution. Don't know if it is applicable though:
"If you look at Eudora Forums > --== WINDOWS EUDORA ==-- > Technical Issues (crashing, error messages, migrating, etc.)
FAQ: Common Error Messages/Codes
Eudora Forums
you will find information about error 550. You can see that there is a knowledgebase article
550 or 553 Relay Denied Error (Windows) entitled 550 or 553 Relay Denied Error (Windows)
This is an error message from your SMTP server informing you that the domain you specified can't be found in the DNS list of the server. Basically they are saying that the domain does not exist.
BUT, as it is 550 that is the error, it may more likely be that you are trying to send the mail using an SMTP server which is not located on the domain of your ISP ie you need to setup a relay personality.
See: Eudora Forums > --== WINDOWS EUDORA ==-- > Network Connectivity / Authentication (Sending/Checking mail issues)
FAQ: Can't send/reply to mail? / How to setup SMTP relay settings
http://eudorabb.qualcomm.com/showthread.php?t=4059"