Computers, Laptops & Tablets Microsoft All About POP Email Errors What it means when your email account talks back to you Share Pin Email Print Robert Nickelsberg/Getty Images Microsoft Microsoft Apple Google Tablets Accessories & Hardware By Heinz Tschabitscher Writer A former freelance contributor who has reviewed hundreds of email programs and services since 1997. our editorial process Heinz Tschabitscher Updated June 09, 2019 If your email account is configured using the Post Office Protocol (POP), you may occasionally get POP error messages while using your account. Here's what those messages mean. POP Status Codes A number of things can go wrong as you download your mail: The server that you usually get your mail from may not answer when you "call" it to deliver your mail.Your password or the server's password is wrong.The server has encountered internal problems. A POP server is clear about its status. With the exception of two words, +OK (which is positive) and -ERR (which is negative), all the words in an error message are delivered in plain English. -ERR mailbox unknown or -ERR invalid password The first thing that can go wrong (apart from the server being down) is the POP server not recognizing your username. Possible reasons for this error include: Typing your username wrong.The database that the server uses to identify users is down.Something is wrong with the storage where mailboxes are kept at your ISP. When you enter your password, that's another opportunity for errors. -ERR unable to lock mailbox A POP mailbox can only be accessed by one incoming connection at a time. If your mail checker is logged in to your email account, your email program cannot get access to the same account at the same time. In such cases, when the mailbox is locked by another process, the POP server returns the "unable to lock mailbox" error. -ERR no such message Once successfully logged into the account, a POP client begins retrieving messages, one at a time. When it requests a message from the server, one negative response is possible: "no such message." The same response can be returned when the email client marks a message for deletion that doesn't exist or that has already been marked for deletion. -ERR some deleted messages not removed When a POP session has ended, all messages marked for deletion are permanently deleted by the server. If the POP server can't remove all messages (possibly because of a resource shortage) it returns a "deleted messages not removed" error.