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.
You need to set your email client to check mail less frequently and it is very important that you ensure you don't have two separate machines checking mail at the same time which is a common cause of this issue.