550 5.1.10 RecipientNotFound: how to fix the Outlook bounce

What 550 5.1.10 means

The bounce comes from Exchange Online, the mail service behind Microsoft 365 work and school email. The full line reads “Recipient not found by SMTP address lookup”: the recipient’s domain is hosted in Microsoft 365, but no mailbox, mail user or contact there has that exact address. Similar codes point to different problems:

Code	What Microsoft says it means
550 5.1.10	The SMTP address lookup didn’t find the recipient in the directory
550 5.1.1	Bad destination mailbox address: a typo, an address that doesn’t exist, or a stale cached entry after a mailbox move
550 5.4.1	Recipient address rejected: Access denied, which is what Directory-Based Edge Blocking returns for unknown recipients
550 5.5.0	Used for @outlook.com and @hotmail.com recipients in a way Microsoft describes as similar to 5.1.10

Check the address shown in the bounce. If it differs from the one you typed, the recipient’s mailbox is forwarding your message to an address that no longer exists, and only they or their admin can fix it.

If you never sent the message

Spammers often forge real addresses in the From field. When their message goes to an address that doesn’t exist, the bounce comes back to you. Microsoft calls this backscatter and says you can ignore or delete these bounces; it doesn’t mean your account was taken over. If you get a lot of them, run a malware scan and ask whoever manages your domain to set up SPF, which helps receiving servers reject forged mail.

Fix it as the sender

A wrong or outdated entry in Outlook’s AutoComplete list is a common cause. The list stores more than the address: it also keeps an internal Exchange identifier, so an entry saved before the recipient’s mailbox moved can still fail even though the address looks right.

Classic Outlook for Windows:

1. Open the bounce message and choose Report > Send Again. If the original had an attachment over 10 MB, resend it from Sent Items instead.
2. Delete the address from the To box.
3. Start typing the name again. When the suggestion appears, point at it without selecting it and click the X (or press Delete) to remove it from AutoComplete.
4. Type the full address by hand, check every character, and click Send.

To clear the whole list instead, go to File > Options > Mail and, under Send messages, click Empty Auto-Complete List. Removing an entry doesn’t delete the person from your contacts.

Outlook on the web: open a new message, start typing the name in To, highlight the wrong suggestion with the arrow keys and press Delete. The web version keeps its own list, separate from the desktop app’s.

If you sent the message to the wrong person and want to pull it back, see how to recall an email in Outlook for when that works. If resending still bounces with the address verified, contact the person another way: their mailbox may have been removed, which you can’t fix from your side.

If you’re the recipient

When people tell you their mail to you bounces with 5.1.10, look for forwarding you set up earlier. In Outlook on the web, check Settings for Inbox rules that forward or redirect to an old address, and turn off account forwarding under Mail > Forwarding if it points to an address that no longer works. Microsoft’s article describes these settings with older menu labels, so the names may differ slightly in the new Outlook.

Fixes for Microsoft 365 admins

If the recipient is in your organization, work through these in order:

1. Confirm the recipient exists and is licensed. In the Microsoft 365 admin center, open Users > Active users, search for the person and check Product licenses. If the user was removed, recreate the mailbox or add a mail contact for their new address. To see whether any object still holds the address, run Get-Recipient <address> in Exchange Online PowerShell.
2. Remove broken forwarding. On the user’s record, open Mail and check Email forwarding. Then review Mail flow > Rules in the Exchange admin center for redirect rules that point to dead addresses.
3. Check the accepted domain type. Under Mail flow > Accepted domains, an Authoritative domain means Exchange Online bounces any address it can’t find. If some mailboxes for that domain still live on an on-premises or other server, the domain needs Internal relay plus a connector to that server. Change this carefully; Microsoft warns that switching the type can cause other mail flow problems.
4. Sync a hybrid directory. If users are synced from on-premises Active Directory, run Start-ADSyncSyncCycle -PolicyType Delta on the Microsoft Entra Connect server and check the recipient again. When a cloud user has no on-premises object, Microsoft’s fix is to create a matching remote mailbox with New-RemoteMailbox and copy the cloud mailbox’s ExchangeGuid to it.
5. Check the MX record. Under Settings > Domains, select the domain and run the DNS check. Microsoft supports a single MX record pointing to Microsoft 365; extra MX records pointing elsewhere can send mail to the wrong system. DNS changes can take up to 72 hours to apply.

For a different Exchange Online error that hits apps and printers sending mail, see 530 5.7.57 Client not authenticated. If the missing recipient is a shared mailbox, confirm it still exists before adding it in Outlook again.

Related guides

• 530 5.7.57 Client not authenticated to send mail: how to fix it
• How to recall an email in Outlook (and when it works)
• How to add a shared mailbox in Outlook (Windows, Mac, Web and Mobile)
• Outlook tips and tricks: 12 ways to manage email faster

Official sources:

• Microsoft Learn: Fix NDR error 550 5.1.10 in Exchange Online
• Microsoft Learn: 550 5.1.10 RecipientNotFound in a hybrid deployment
• Microsoft Learn: The Outlook AutoComplete list