During a POP session there is a lock file that is put on the account so that only one device can POP at a time. After the POP session is over, the lock file is removed from the mail server to allow the next POP session. If you're checking your mail again during the first POP session, then this error will occur.

The best way to avoid this, is to not send/receive so quickly. Set the frequency that your mail program automatically checks email to at least 5 minutes or more.  If you click the Send/Receive button twice in a row, you may get this error also.

Only use a POP3 type connection for one email device, like your desktop computer program (Outlook or something similar). Then use an IMAP type connection for all other devices, like smart phones or tablets.

There is also a known issue on the iPhone where the email program on the iPhone doesn't properly release the lock on the account, which will lead to the lock error. We recommend only using the IMAP connection type on the iPhone.