Is 550 5.1.10 permanent or temporary?

550 5.1.10 is a permanent (5.x.x) failure — the server will not accept the message as-is, and retrying without changing anything won't help. You need to fix the underlying cause.

Is this my problem or the recipient's?

It depends on the cause — the address is wrong or no longer exists is the most common. Work through the causes below to find which applies to you.

How do I check my sending setup?

Run a free EmailHealth check on your domain — it verifies SPF, DKIM, DMARC, blacklist status, and more in a couple of seconds, which covers the most common causes of delivery failures.

550 5.1.10 — Recipient not found (Microsoft 365 / Outlook)

550 5.1.10 is a permanent SMTP reply meaning recipient not found (microsoft 365 / outlook). Microsoft explains that the SMTP address lookup did not find the recipient. For @outlook.com and @hotmail.com addresses the same problem can appear as 550 5.5.0 "Requested action not taken: mailbox unavailable".

What 550 5.1.10 means

Microsoft explains that the SMTP address lookup did not find the recipient. For @outlook.com and @hotmail.com addresses the same problem can appear as 550 5.5.0 "Requested action not taken: mailbox unavailable".

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This is a 5.x.x permanent failure — the message will keep bouncing until you fix the cause. Retrying the same message unchanged won't help.

Common causes

📨

The address is wrong or no longer exists

Most common

A typo, or a person who left the company and whose mailbox was removed.

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Your list has old or guessed addresses

Common

Pattern-guessed or bought lists produce many 5.1.10 bounces at once, which also hurts your sender reputation.

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The recipient domain moved to Microsoft 365 but the user was not set up

Less common

The domain points its MX at Microsoft, but this address was not created there.

How to fix 550 5.1.10

1
Check the exact address
Confirm the spelling with the recipient, or find a current address another way.
2
Remove the address from your list
Do not retry. Suppress it so you do not send to it again.
3
Verify lists before large sends
A high rate of 5.1.10 bounces is a reputation problem. Verify addresses before a campaign, especially guessed ones.

Confirm your domain is set up correctly

Most delivery failures — permanent or temporary — trace back to authentication or reputation. Run a free EmailHealth check to confirm SPF, DKIM, DMARC, blacklist status, and TLS are all healthy for your sending domain. If you send in volume, free daily monitoring will alert you before a small issue becomes a flood of bounces.

Recipient not found (Microsoft 365 / Outlook)