This sounds like it could be an authentication error on the server itself, but it would be best to do step by step troubleshooting.
Firstly, ensure that you are able to log into webmail, this way you can establish if the mail server is working correctly. Also ensure that your DNS records are setup correctly to point your MX (mail) records to the same server IP as your webmail login directs you to. You can do an NSLOOKUP on the server name to verify it's IP address (will usually be in the format XXX.aserv.coza).
Some mail clients don't like passwords with certain characters, so make sure you are using a client friendly password. Try simplifying your mailbox password first in cPanel and then retesting.
It's hard to give further advice without knowing the specifics of your setup. Drop me a mail on firstname.lastname@example.org so I can try to give you further help with this (if the steps above do not help). You can also check your setup against this guide: