What does "mailbox unavailable" mean?

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Summary: Mailbox unavailable is a common message in returned email or "bounces" that actually says very little other than that the email could not be delivered.

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Well, on one hand it means exactly what it says: your friend's mailbox isn't available, and your message could not be delivered.

In fact, that's exactly, and the only thing it's saying.

But we can make some guesses as to what it might mean.

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"Mailbox unavailable" is a frustratingly vague message. Quite literally it's telling you that the mail could not be delivered, without giving you any clue as to why. None. Unless there's additional text in the error message saying more, there's simply no way to know exactly why it failed.

It gets worse. Different mail systems use different error texts to report the same errors. So whatever might cause a "mailbox unavailable" from one system, might well generate a completely different (and hopefully more useful) message from another.

So, all we can do is guess.

But, we can make some educated guesses.

First let me be clear: I'm NOT saying that any of these did happen. We don't know. These are simply possibilities, nothing more.

Of course, you should start by being absolutely positively certain that you have the email address exactly correct. Any error could have you sending to a non-existent account, which could easily result in this error.

If you are certain that your friend should still have his email account, then it's likely to be a temporary problem at his end. More often than not, it's something as simple as an account that has reached its quota and simply doesn't have room for any more mail. Your friend would have to download or delete email in his account to make room for new messages.

Some services use "Mailbox unavailable" as an error message to reject suspected spam. If your email somehow looks like spam, or your email address has somehow become associated with being a spammer, this could be the system rejecting your message.

It's possible that the server on which your friend receives email has been misconfigured, and is erroneously rejecting mail to him for a variety of reasons.

It's possible that your friends account was suspended or deleted for abuse. Sometimes this means that he was doing something he shouldn't have, but more often than not it means that his account was hacked or spoofed and taken over by someone who did. Once they got caught, the account might be closed or disabled.

Perhaps your friend closed his account without telling you. This happens, intentionally or by accident. You would need to find another way to contact your friend that doesn't involve this particular email account.

While those are all things to look at and rule out, what this all boils down to is that basically any reason for email not being delivered could, in fact, cause this message.

In your shoes - I would contact your friend via some other means.

Related:

• Why does email bounce? Email can bounce for many reasons. I'll look at several of the most common mail bounce messages, and try to interpret what they really mean.

• Can I count on mail to a bad address bouncing back? If you send email to an invalid address or a closed account, usually you'll get a bounce back. Usually. The problem is that you cannot count on bounces.

• Are free email services worth it? Free email services and accounts are convenient and ubiquitous. But free email services aren't the right place to keep your important information.

Article C3738 - May 21, 2009