I had a weird problem earlier with a domain on my Raq4r. The user was trying to get mail, but they kept getting a poplock error. So I went into /var/spool/mail and sure enough, it was there, so I removed the lock. What would cause this? I verified the username and password that he was using, and it was fine. This went on all day, but I thought poplock was supposed to release on it's own.

Restart your popserver. There could be a proccess waiting to get to that folder, once there it locks and then for whatever does not unlock.

I have found the some mail clients can cause this problem or if people use two mail clients at the same time, e.g. PINE and Outlook, or Pine and Web-based program.

Restart the mailserver and associated programs (imap,pop). If the problem persists, then ask the client about what mail program they are using.

I don't know what kind of POP server running in Cobalt, but if it running qpopper.. you will see some poplock problems. Yes, poplock suppose to clear itself automatically in most cases. But if you have a big number of email or somebody send you a huge attachment, that will almost automatically lock the mailbox.
I would try changing qpopper to cucipop if the problem keep showing.

Well, whatever was wrong just got worse. After I deleted the lock yesterday it worked just fine. But today it gave me an error about the mailbox being in the incorrect format. So, I just deleted it and re-created it.