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550 5.1.10: Recipient not found (Microsoft)

Microsoft 365 couldn't find the recipient's SMTP address in the destination tenant. The address doesn't exist.

Bounce type
Hard bounce (permanent)
What to do
Suppress the address
Returned by
Microsoft (Outlook.com, Microsoft 365)

What 550 5.1.10 means

Microsoft documents 5.1.10 as 'Recipient not found': the SMTP address lookup in the recipient's Microsoft 365 organization didn't find the address. It's Microsoft's flavor of 'user unknown'.

If you're emailing a company that runs on Microsoft 365, this is the bounce you'll get for a wrong or deleted address. It's permanent and counts as a hard bounce against your sender reputation, the same as 5.1.1.

What the bounce looks like

The wording providers document for this code. Your bounce may add an IP address, a session ID or a help link after it.

Microsoft
550 5.1.10 RESOLVER.ADR.RecipientNotFound; Recipient not found by SMTP address lookup

Why cold emailers see 550 5.1.10

  • The employee left and their mailbox or alias was removed.
  • A wrong guessed format for that company's addresses.
  • An address that only ever existed in a data vendor's database.
  • A shared mailbox or distribution group that was renamed or removed.

How to fix 550 5.1.10

  1. Suppress the address. Don't retry and don't keep it in sequences.
  2. Re-verify the rest of the contacts from the same source and company; stale data tends to cluster.
  3. If you think the person is still there, find their current address through another channel rather than guessing variants. Guessing generates more hard bounces.
  4. Track hard bounces per data source. If one vendor's list produces a cluster of 5.1.10s, stop sending to the rest of it until it's re-verified.

Retry or suppress?

Suppress permanently. A 5.1.10 will never succeed on retry.

Reading the code

The basic code 550 means "Mailbox unavailable or message refused (permanent)" under RFC 5321. In 5.1.10, the 5 is the class (permanent failure) and the 1 is the subject (addressing). The detail number 10 is registered in RFC 3463 / the IANA registry as "Recipient address has null MX".

Class 5
Permanent failure
Resending the same message won't work. Something about the message, the recipient or the sender has to change first.
Subject 1
Addressing
A problem with the sender's or recipient's address: syntax or validity.
Detail 5.1.10
Recipient address has null MX

Check the rest of your setup

Paste the headers of a message that did get through into the email header analyzer to confirm SPF, DKIM and DMARC pass, and check your domain with the SPF, DKIM & DMARC checker. The deliverability checklist covers the rest of a healthy sending setup.

Got a different bounce? Paste it into the SMTP error code lookup to decode it.

FAQ

Is 550 5.1.10 a hard bounce or a soft bounce?

550 5.1.10 is a permanent failure (a hard bounce): the receiving server won't accept this message if you send it again unchanged. Here the problem is the address itself.

Should I remove the contact after a 550 5.1.10 bounce?

Yes. Suppress permanently. A 5.1.10 will never succeed on retry.

What does 5.1.10 mean in the error code?

In 5.1.10, the 5 is the class (permanent failure) and the 1 is the subject (addressing). The detail number 10 is registered in RFC 3463 / the IANA registry as "Recipient address has null MX".

Related SMTP codes

See all SMTP error codes

Sources

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