I have had the same problem since the New Year. Outgoing mail is fine, so my SMTP and email account must be OK, but it always fails on authentication with macmail's incoming pop server. I can access my mail via the web, so I am getting mail! I have tried re-installing Mail, deleting all plists first, but no difference. I have tried on two other newly built macs - same problem. I have also tried resetting my macmail password - didn't help. I set up Mail to access MacMail via IMAP instead, but I'd rather get the POP3 fixed. I emailed both Macmail support and their parent hoster Talia twice, and have not got a reply.
I am stunned that there aren't thousands of others in the same boat - there doesn't seem to be much on any forums from other users with this same problem. I am convinced the problem is at their end.
I'm running 10.4.4 on an imac G5, and have had no problems other than this with my macmail account I've had for around 4 years.