Error message: 550 5.5.0 Requested action not taken: mailbox unavailable

Did you get an email back with the message 550 5.5.0 Requested action not taken: mailbox unavailable? This means the receiving mail server has permanently rejected your message because it can't or won't use the mailbox. A code that starts with a 5 is permanent: resending only helps once the cause has been fixed.

This article covers this one specific message. How to read an error message (bounce) and what all the other codes mean is explained in Mail delivery failed: how to read the error message and what the codes mean. The extended code 5.5.0 is a general one. For an address that doesn't exist or a full mailbox, you'll more often see 550 5.1.1 or 552 5.2.2; those are covered in that article too.

Possible causes

• The address doesn't exist or contains a typo. Check the spelling, or ask the recipient for the correct address through another channel.
• The recipient's mailbox is full or blocked. In that case the recipient needs to free up space or contact their own provider. Let them know through another channel.
• You're sending mail from your website to your own domain, while your mail is hosted elsewhere. If your mail is hosted with Microsoft 365 or Google Workspace, for example, our server delivers mail from your website to your own domain locally and you get this message, often followed by or not local. Read Sending mail via PHP doesn't work, the MX records point elsewhere.
• The recipient is rejecting your mail due to their own policy. Read the text after the code carefully: it usually states the reason.

Still stuck? Create a ticket via the Support Center with the complete error message, the recipient's address and the time it happened.

Detailed explanation with screenshots: I can't send email in the DirectAdmin manual.

Want to learn more? Take the free course Getting your email in order, lesson 6.2 Common error codes explained.