When attempting to check your email using a standalone email client like Eudora, Thunderbird or Outlook, you may encounter the error message "Mailbox is locked by POP server." This problem usually occurs when some other program using the same protocol has recently accessed your mail and has not yet released it. In some cases, this problem may also be caused by software incompatibilities.
POP stands for Post Office Protocol, a standard originally developed in the mid-1980s for sending electronic mail. As of February 2012, the current version is POP3, which works as a standalone protocol and is used by the vast majority of email servers. This protocol supports simple download and delete options and usually moves all messages to your computer when you access the mail server. Each attempt to access the server is considered a separate POP session, and most servers do not support multiple POP sessions for any given mailbox.
Checking Email Too Often
If you repeatedly access the POP server by checking your mail several times within a few minutes, you may accidentally lock up the mailbox. This also sometimes happens when inconsistent Internet connections cause your mail program to send multiple requests to the server. Waiting a few minutes before connecting again usually clears up this type of mailbox lock.
You may also receive the "Mailbox is locked by POP server" message if you use a third-party program to check your mail for spam, viruses or other undesirable material. In this case, your mail program and your email checker are both attempting to access the mail server at the same time, causing duplicate POP sessions. You can correct this problem by changing the mail access settings in the third-party program or discontinuing it.
Users who access their email on smartphones like the iPhone or Android systems may see a locked mailbox message if they also read email on their home computers. This problem is most likely to occur if both devices are left on and allowed to check for new mail automatically, or when you switch back and forth from the computer to the smartphone in a short period of time. Closing one mail program or waiting a few minutes between using devices should correct this problem.