That's great information!
I got that set up by going to Edit -> Mail and Newsgroup Account Settings
Select "Add Account" and "Unix Mailspool (Movemail)" is right there.
Note I said move the mail. That means the mail is deleted. Use at your own risk of losing mail.
Thunderbird has a setting to leave messages on the server -- even with a Unix Mailspool account -- but Seamonkey doesn't.
I haven't tested it to see if it works.