There could be many different possible causes, and you may want to contact support via support.lunarpages.com and submit a ticket.

When contacting support, be sure to provide:

What do you have typed in the incoming (pop3) server name field?

What do you have typed in the outgoing (smtp) server name field?

What do you have typed in the incoming (pop3) username field?

What do you have typed in the outgoing (smtp) username field?

What ports do you have set for POP3/IMAP and SMTP server?
Also, are you using Norton Utilities or Norton Anti-Virus?

NOTES:
Ensure that you have SMTP Authentication Enabled.
Ensure that you have SSL Authentication Disabled.

Please also provide the following information:

1. The mail program you are using.
2. The exact error message you are receiving. (Outlook and Outlook Express mail clients will provide an error code with each error that starts with 0x800)
3. The e-mail address and a temporary password (you can change this in cPanel/plesk/LPCP, then change it back once we are done testing) for the e-mail account with the issue so we can try to duplicate this.