Mail Not Sending using POP3

[Tara]

I am having a problem sending to some email addresses. I keep getting a
returned message stating:
----- The following addresses had permanent fatal errors -----
<'(e-mail address removed)'>
(reason: 550 Host unknown)

----- Transcript of session follows ----- 550 5.1.2
<'(e-mail address removed)'>... Host unknown (Name server: charter.net': host not
found)

I called my ISP and they stated that it had to do with the XP Firewall
blocking POP3 email. Anyone have suggestions on how to correct this? Thanks!