This error comes up only when using a POP3 mail client.
When a POP3 mail client logs into a mailbox (ie- your mailbox), it has to lock the mailbox to ensure nothing changes while it’s attempting to read your messages. If for some reason your POP3 sessions dies while your mailbox is locked (if you lose your internet connection, or your computer crashes), then the mailbox may end up in a state where it doesn’t get un-locked for the next time you check your mail.
What happens then, is when you try to check your mail again (after recovering from your error), it will try to lock the mailbox again, but the mail server will return this error message- ie- it was un-able to lock the mailbox because it is already locked from a previously failed session.
The *only* way to get around this, is to leave your mail program for 15 minutes, and the mailbox lock will eventually timeout on it’s own.
This won’t affect any of your mail deliveries (ie- no mail will be lost). This only keeps you from being able to check your mail.