Microsoft Outlook error 0x800CCC92

I get the following error when trying to retrieve e-mail from our POP3 Postfix mail server (on Solaris 5.8). We're also using Sendmail to interact with the PreciseMail spam filtering system:

Task 'mail.example.com - Sending and Receiving' reported error (0x800CCC92): 'Your e-mail server rejected your login. Verify your user name and password in your account properties. Under Tools, click E-mail accounts. The server responded: ??RR /usr/mail/.thomas.pop lock busy! Is another session active? (11)'

Basically, it indicates that the previous POP3 connection terminated in an unclean manner.

Links:
Telnet - POP Commands (retrieving mail using telnet)
POP lock busy

Looking at the contents of /usr/mail ("ls -la /usr/mail") you will see that there are at least one (and possibly multiple) .pop files in the directory (".username.pop").