How to Understand POP Email Errors

What they mean and what to do about them

People checking email at a coffee shop

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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

Some things can go wrong in this process of downloading mail. The server that you usually get your mail from may not answer the call at all. Or maybe your password is wrong (but maybe the server's password is wrong, due to some software glitch). The server could also run into some internal problems and reply with an error code.

Fortunately, a POP erver is very clear about its status. It basically knows two replies: the positive +OK and the negative -ERR. Of course, this is a bit unspecified if you want to know what has gone wrong.

As it turns out, +OK and -ERR is about all the new code you have to learn if you want to understand POP error messages. All the rest is the standard code: human language. Apparently, the Post Office Protocol was designed by human beings for human beings. More detailed information about an -ERR server response is given in plain English, following the -ERR message. While POP servers are not required to offer this additional information, most do.

POP Error Messages

The first thing that can go wrong (apart from the server being down altogether) is the POP server not recognizing your username. Maybe it was typed in wrong, maybe the database that the server uses to identify users is down. Maybe a flood has destroyed all the storage where mailboxes are kept at your ISP.

When a POP server does not recognize a username, sometimes due to misspelling or case sensitivity, it will typically reply with -ERR mailbox unknown.

Just after the username comes the password, and another chance for errors. Errors, that's right, because apart from the password not matching the username (-ERR invalid password) the POP server can run into another problem. A POP mailbox can only be accessed by one incoming connection at a time. If your mail checker has already 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 already locked by another process, the POP server returns: -ERR unable to lock mailbox.

Once successfully logged into the account, a POP client will usually begin retrieving messages, one at a time. When it requests a message from the server, one negative response is possible: -ERR no such message. Looks like the client has a problem. The same response can be returned when the email client tries to mark a message for deletion that does not exist (or has already been marked for deletion).

When a POP session is ended, all messages marked for deletion are usually permanently deleted by the server. If the POP server cannot remove all messages (possibly because of a resource shortage) it returns an error: -ERR some deleted messages not removed.

See For Yourself

Since the Post Office Protocol is so simple, there are only a few things that can go wrong, and only few error messages. All errors returned by a POP server are indeed messages and not mere, cryptic codes.

If your email program turns these meaningful error messages into non-descriptive error boxes, it is probably best to try it yourself. Fire up a DOS prompt and telnet directly into your email account. Type telnet <POP server name> <port>. Usually, the port used for POP is 110. A typical command could look like this, for example: telnet 110.

When the server greets you with a happy +OK, follow the process as described in The Post Office Protocol and you should be able to identify the error. At least, if everything works fine, you know that the problem is really with your email client, not your email server.