Fixes / Outlook · Checked

Outlook "Delivery has failed" 550 5.1.1: meaning and fix

Short answer

Error 550 5.1.1 means the receiving mail server rejected your email because it couldn't find a mailbox at that address. Usually the address is misspelled or no longer exists. In Outlook, a stale AutoComplete suggestion is another common cause. Check the address in the bounce message, delete the suggestion, type the full address by hand, and send the message again.

What this message means

The bounce report (NDR) comes from the recipient's mail server or from your own Exchange server. It means the server looked up the address and found no mailbox, alias or group with that name. The code is permanent (the 5 at the start), so Outlook won't retry. Sending the same message again won't help until you change something. The fault is almost always the address itself: a typo, a person who has left the company, or a domain that moved to a different mail system. In Microsoft 365 there's another possibility. Outlook may have saved an old internal Exchange identifier for the contact, so the address looks right but points at a mailbox that no longer exists.

Common causes

CauseHow to tell
The email address is misspelled or incompleteThe bounce message quotes the address it tried. Compare it letter by letter with the correct address, checking the domain especially (for example .co vs .com, gmial vs gmail). Many servers add text such as "The email account that you tried to reach does not exist" or "User unknown".
The mailbox was deleted, renamed or the person left the organizationThe address worked in the past but now bounces every time. Other people at the same domain still get your mail.
Outlook's AutoComplete (nickname cache) holds a stale entryThe bounce shows a strange address beginning with IMCEAEX- or containing /O= or /CN=, and often RESOLVER.ADR.ExRecipNotFound or RESOLVER.ADR.RecipNotFound. Typing the full address by hand works, but picking the suggested name fails. This often happens after a mailbox migration, or when you reply to old messages imported from a .pst file.
The recipient forwards your mail to an invalid addressThe address in the bounce differs from the one in your Sent Items message. The recipient has an Inbox rule, a forward or a mail flow rule sending copies to an address that doesn't exist.
The recipient's domain has wrong or stale MX recordsEveryone who writes to anyone at that domain gets 550 5.1.1, even with correct addresses. This often starts right after the domain moved to Microsoft 365 or another mail provider.
Backscatter or a compromised accountYou get bounces for messages you never sent, and they're not in your Sent Items (backscatter, safe to ignore). If Sent Items does contain messages you didn't write, someone else is using your account.

How to fix it

1. Check the address in the bounce and resend

  1. Open the bounce message and find the address listed under "Delivery has failed to these recipients or groups".
  2. Compare it with the correct address, checking the part after the @ especially.
  3. Open the original message in Sent Items and resend it, or start a new message.
  4. Type the full address yourself instead of picking a suggestion, and send.

2. Remove the bad AutoComplete suggestion (classic Outlook for Windows)

Classic Outlook for Windows (Microsoft 365, 2021, 2019, 2016)

  1. Open a new email and type the first few letters of the recipient's name in the To box.
  2. When the suggestion appears, hover over it (don't click it) and click the X next to it, or highlight it with the arrow keys and press Delete.
  3. Type the complete email address by hand and send.
  4. If that doesn't help, clear the whole list: File > Options > Mail. Under Send messages, click Empty Auto-Complete List and confirm with Yes.
  5. Or close Outlook, press Windows+R, type outlook.exe /CleanAutoCompleteCache and press Enter.

3. Clear suggested contacts in new Outlook, Outlook on the web or Microsoft 365

New Outlook for Windows, Outlook on the web, and classic Outlook with Exchange Online mailboxes

  1. Since Microsoft retired Contact Masking in 2026, clicking X on a suggestion in Exchange Online mailboxes may not stick. The removed name can come back.
  2. In new Outlook or Outlook on the web: Settings (gear icon) > General > Privacy and data. Under Suggested people, clear the data.
  3. Work or school account: go to myaccount.microsoft.com > Settings & Privacy > Privacy. Under Data options, expand Manage contact suggestions and discovery, then select Clear data under Discovered contacts.
  4. Personal Microsoft account: go to account.microsoft.com/privacy and turn off or clear People suggestions.
  5. Type the recipient's full address by hand and send again.

4. Confirm the address with the recipient

  1. Contact the person another way (phone, chat, text) and read them the exact address you used.
  2. Ask whether the address changed, for example after a new job, a name change or a company rebrand.
  3. If the address is correct but mail still bounces, ask them to check for Inbox rules or forwarding to another address.
  4. Update or delete the saved contact in Outlook so the old address isn't suggested again.

5. Admin checks: X500 addresses, forwarding and MX records

Exchange Online / Microsoft 365 and hybrid Exchange admins

  1. If the bounce shows an IMCEAEX- address after a migration, add the old LegacyExchangeDN as an X500 proxy address on the recipient's mailbox. Users can also clear their AutoComplete entry.
  2. In the Microsoft 365 admin center, check Health > Service health and the Message center for known issues.
  3. Check forwarding on the recipient's mailbox, their Inbox rules and your mail flow (transport) rules for invalid destination addresses.
  4. Make sure the domain has one MX record pointing to <domain>.mail.protection.outlook.com (for Exchange Online).
  5. Test the domain with the Inbound SMTP Email test at testconnectivity.microsoft.com. DNS changes can take up to 72 hours to propagate.

6. Check for a compromised account or backscatter

  1. Open Sent Items and look for messages you didn't send.
  2. If there are any, change your password at once and scan your computer for malware.
  3. Check Inbox rules and forwarding for anything you didn't set up, and tell your IT admin.
  4. If Sent Items is clean, the bounces are backscatter from spammers faking your address and you can ignore them.

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Still not fixed?

If the address is definitely correct, you typed it by hand and clearing AutoComplete didn't help, the problem is on the receiving side. Forward the full bounce message, including the Diagnostic information for administrators section, to the recipient by another channel or to their IT department. It names the server that rejected the mail. If the rejecting server is your own organization's (the bounce names your domain), send it to your Microsoft 365 or Exchange admin. They can run a message trace and check the recipient's proxy addresses.

Questions people ask

Does 550 5.1.1 mean the email address doesn't exist?

Yes, in most cases. The receiving server couldn't find a mailbox at that exact address, usually because it's misspelled, was deleted or has changed. In Microsoft 365 organizations it can also mean Outlook used an outdated cached entry for a valid person.

Why do I get 550 5.1.1 when the address is correct?

Outlook's AutoComplete may hold an old internal Exchange identifier for that contact, often after a mailbox migration or when you reply to an imported old email. Delete the suggestion, type the full address by hand and send again. If it still fails, the recipient may be forwarding to a bad address, or their domain's mail setup may be broken.

Will Outlook retry sending the message automatically?

No. Codes starting with 5 are permanent failures, so nothing is retried. You have to fix the address and send the message again yourself.

What is the difference between 550 5.1.1 and 550 5.1.10?

Both mean the recipient wasn't found. 5.1.10 (RESOLVER.ADR.RecipientNotFound) is the Microsoft 365 version, usually raised when the address isn't found in the recipient's Exchange Online directory. 5.1.1 is the more general code that most mail servers use, including Gmail and on-premises Exchange.

I got 550 5.1.1 bounces for emails I never sent. Am I hacked?

Check your Sent Items. If the messages are there, your account is probably compromised: change your password and tell your admin. If they're not there, spammers faked your address (backscatter) and you can safely ignore the bounces.

Sources

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