Microsoft 365 "550 5.1.10 RESOLVER.ADR.RecipientNotFound": how to fix it

What it means

The email reached a Microsoft 365 organization, but Microsoft could not find a mailbox, group or contact with that address. The bounce usually contains:

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

RESOLVER.ADR is the part of Exchange that resolves addresses. It is a permanent error: resending the same message to the same address will fail again. For Outlook.com and Hotmail addresses, Microsoft returns a similar error, 550 5.5.0 Requested action not taken: mailbox unavailable.

A related error, 550 5.4.1 Recipient address rejected: Access denied, means the address was refused even earlier, at the SMTP connection, by Directory-Based Edge Blocking.

Why it happens

On the sender's side

• A typo in the address, or an old address saved in Outlook's Auto-Complete list.
• The person left the company and their mailbox was deleted.
• Backscatter: you never sent the message. A spammer used your address as the sender, and the bounce came back to you. Microsoft says this is harmless, but a lot of it suggests your domain is being spoofed.

On the recipient's side

• The user exists but has no Exchange Online license, so there is no mailbox.
• A broken forwarding rule: the recipient (or an admin rule) forwards messages to an address that no longer exists. The address in the bounce is then different from the one you typed.
• Hybrid or partial migration: some users are still on another server, but the domain is set as Authoritative in Microsoft 365, so unknown users are rejected instead of being relayed.
• Directory sync problems: the user exists on-premises but was not synchronized to Microsoft 365.
• Wrong MX record, for example the domain still points to Microsoft 365 after the company moved to another provider, or there are several MX records. Microsoft does not support more than one MX record for a domain enrolled in Exchange Online.

How to check

1. Compare the address in the bounce with the address in your Sent Items. If they differ, a forward on the recipient's side is broken.
2. Check the recipient domain's MX with dig +short MX example.com or the Delivmail domain scanner. A Microsoft 365 domain points to example-com.mail.protection.outlook.com (or a .mx.microsoft host).
3. If you are the recipient's admin, search the address in Exchange Online:

   Get-Recipient -Identity "user@yourdomain.com" -ErrorAction SilentlyContinue
   Get-Recipient -ResultSize Unlimited | Where-Object {$_.EmailAddresses -match "user@yourdomain.com"}
   Get-AcceptedDomain | Format-Table DomainName,DomainType

4. Run a message trace for the recipient address to see whether the message arrived and where it was rejected.
5. If you receive bounces for messages you never sent, check your domain's SPF, DKIM and DMARC by sending a test to the Delivmail home page address.

How to fix it

If you are the sender

1. Remove the address from Outlook's Auto-Complete list (start typing it, then click the X next to the suggestion), type it again in full and resend.
2. If it still bounces, contact the person another way (phone, website form) to confirm their current address.
3. For newsletters, remove the address from your list: repeated sends to non-existent addresses damage your reputation (see clean your mailing list).
4. For backscatter, do not reply. Publish a DMARC policy to make spoofing of your domain harder: someone sends fake emails with your domain.

If you are the recipient's admin

1. Create or restore the recipient: add the user, assign an Exchange Online license, or add the address as an alias on the right mailbox. A deleted user can be restored from Users > Deleted users in the Microsoft 365 admin center for a limited time (30 days).
2. Fix forwarding: correct or remove Inbox rules, mailbox forwarding (Users > Active users > Mail > Email forwarding) and mail flow rules that redirect to dead addresses.
3. Hybrid or shared domains: if some users live on another server and you have a connector to it, set the accepted domain to Internal Relay (Exchange admin center, Mail flow > Accepted domains). Without a working connector, this breaks mail flow, so check first.
4. Directory sync: on the Microsoft Entra Connect server, run a delta sync, then check the recipient again:

   Start-ADSyncSyncCycle -PolicyType Delta

5. Fix the MX record: keep a single MX pointing to Microsoft 365 if the domain is hosted there, or remove the domain from Microsoft 365 if it moved elsewhere. In Settings > Domains, use Check DNS.

Check that it is fixed

• Ask someone outside the organization to send a short test to the address, or send one yourself, and check the message trace.
• After an MX change, wait for DNS propagation (often a few hours, up to 72 hours according to Microsoft) before concluding.
• Resend the original message only once the address is confirmed.