Microsoft 365 Exchange Error Code 5.5.0 – 550 5.5.0 Requested action not taken: mailbox unavailable

In this post, let’s have a look at the error message “5.5.0 – 550 5.5.0 Requested action not taken: mailbox unavailable” that you get when working with Microsoft 365 Exchange Online.

Error Message

5.5.0 – 550 5.5.0 Requested action not taken: mailbox unavailable

Reason

The recipient’s domain is @hotmail.com or @outlook.com and it wasn’t found by SMTP address lookup.

Microsoft 365 Exchange Error Code 5.5.0 – 550 5.5.0 Requested action not taken: mailbox unavailable is a common error that can occur when sending an email through Microsoft 365 Exchange. This error message indicates that the recipient’s mailbox is not available, and the requested action cannot be taken.

Here’s an example to help understand the issue:

To: [email protected]
From: [email protected]
Subject: Test Email

This is a test email.

In this example, test2 is trying to send an email to test1 at the email address [email protected]. However, she receives the following error message:

Delivery has failed to these recipients or groups:
[email protected]
The email address you entered couldn't be found. Please check the recipient's email address and try to resend the message. If the problem continues, please contact your helpdesk.
Diagnostic information for administrators:
Generating server: developerpublish.com
[email protected]
Remote Server returned '550 5.5.0 Requested action not taken: mailbox unavailable'

Because test1’s mailbox is not available, this error message indicates that the email could not be delivered. A mailbox may be unavailable for a variety of reasons, including the following:

  • The mailbox of the recipient is full and cannot accept new messages.
  • The mailbox of the recipient has been deleted or disabled.
  • The email address of the recipient is incorrect or misspelt.
  • The recipient’s email server is having technical difficulties.

Solution

You may need to try the following solutions to resolve this error:

  • Check the email address of the recipient for accuracy and spelling mistakes.
  • Check to ensure that the recipient’s mailbox is not full or disabled.
  • Confirm the status of the recipient’s mailbox with them or their IT department.
  • If the problem is only temporary, try sending the email again later.