Because of the way POP3 works, it's impossible to be logged in to the same mailbox with two sessions at the same time: to do so would cause some of your emails to disappear. So, to prevent this and ensure that your emails are delivered promptly, you'll see the message:
-ERR Mailbox still locked: you're checking mail too frequently.
...if you log in to your mailbox less than 2 minutes after your last login. If your mailbox is held open for more than two minutes and you try to check your mail again, you'll see the message:
-ERR Mailbox already locked.
If this second message persists (happens a number of times in a day) then you may want to contact customer services to see if there's some external factor affecting your mailbox access.
If you're getting the first message, all you need to do is set your email client to check mail less frequently.
Do ensure that you don't have two separate machines checking mail at the same time which is a common cause of this issue.
