550 Mailbox unavailable error: causes and fix

Introduction

The 550 Requested action not taken: mailbox unavailable message means the remote server permanently rejected delivery to that mailbox.

This may happen because the address does not exist, the mailbox is disabled, the mail route is wrong or the receiving server is applying a specific policy.

What you should review

1. The recipient address

Check for typos, wrong domains or old saved addresses.

2. Whether the mailbox really exists

The account may have been deleted, renamed or disabled.

3. The destination domain

Review whether the domain resolves correctly and whether its mail records point to the right server.

4. Aliases and forwarders

Some visible addresses depend on internal redirects that may no longer be working.

How to act

1. Keep the full bounce message.
2. Verify the address with the recipient.
3. Test with another mailbox in the same domain if one exists.
4. If the issue affects your own domain, review mailboxes, MX records and routing.

Useful tips

1. Do not assume the issue is always on your side

   Very often this error is caused by something on the receiving server side.

2. Understand the difference between permanent and temporary

   A 550 is usually permanent. It is not the same as a temporary deferral.

3. Check for recent DNS changes

   If the domain changed provider or server, the mail route may now be broken.

Frequently asked questions

Is it the same as No Such User Here

They are very similar. Both usually mean the server cannot deliver to that mailbox, even though the exact wording may vary.

Can it be caused by a suspended account

Yes. A disabled account or blocked mailbox can trigger this rejection.

Should I simply try again later

Only if you suspect a recent change. If the mailbox does not exist, retrying will not help.

Conclusion

The 550 Mailbox unavailable error usually means the receiving server cannot accept that message for the specified mailbox.

The best way to solve it is to confirm the address, mailbox existence and domain configuration before resending.