Fix email delivery issues for error code 5.1.10 in Office 365

This topic can help you fix email problems that are associated with error code 550 5.1.10 if you see this in a non-delivery report (NDR). Mail flow problems can be frustrating, but we have solutions you can try in order to get your message sent.

email user symbol

I sent a message, what can I do to fix this?

Administrator symbol

I'm the Admin, what can I do to fix this?

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Why did I get a non-delivery report?

I got this non-delivery report but I didn’t send a message to the address shown

It’s possible that a spammer or someone else inappropriately used your email address to send a message, but the person it was sent to doesn’t exist, so an NDR was sent back to you. This is called backscatter. This doesn’t necessarily mean your email account was hijacked; it’s possible the spammer is just putting your email address in the From part of the message they sent. However, to help ensure your email account hasn’t been compromised check your Sent folder for any messages that you know you didn’t send. If your Sent folder contains only messages you know you sent, the NDR you received could be backscatter, and you can ignore it. If you find messages in your Sent folder that you did not send, it’s possible your email account was compromised. To fix it, do the following two tasks:

  • Run an anti-malware scan on your computer.

  • Change your email account password. Make sure you use a strong password (8 or more characters long, including a combination of upper and lower-case text, symbols, or numbers).

You can find out more about backscatter at Backscatter messages and EOP.

email user symbol

I sent a message and got error code 5.1.10 in an NDR. What can I do to fix this?   

Here are 3 solutions you can try. You might not need all of them to fix the problem. If the steps here don’t help you, contact your email administrator and refer them to the administrator help section of this topic so they can help you.

  • Solution 1: Make sure the recipient's email address is correct.    Check for correct spelling. Delete and retype the address then send the same message again.

    Resend the message by using Outlook
    1. Open the NDR message you received.

    2. From the Report tab choose Send Again.

      Send again button in the Outlook report tab
    3. Select the email address in the To box, then press the Delete key on your keyboard.

    4. Retype the entire address. If you’re prompted with a suggested address do not select it.

    5. Choose Send.

    Resend the message by using Outlook Web App
    1. Select the NDR message you received.

    2. In the message area, just below the To line, click on the link where it says To send this message again, click here.

      Send this message again link in Outlook Web App
    3. In the To line of the new message click on the delete icon that appears next to the email address.

    4. Retype the address completely. If you're prompted with a suggested address do not select it.

    5. Choose Send.

  • Note: If your original message contains an attachment larger than 10 MB, the option to send the message again from the NDR message will not be available. In this case, you'll have to locate the original message in your Sent folder to resend it.

  • Solution 2: Update the recipient's email address from your auto-complete list.    Your mail program may have used out-of-date address information it has stored for the person you’re sending to. You can remove this out-of-date address information from your address Autocomplete list by doing the following:

    1. Create a new email message. In Outlook select New Email; in Outlook Web App select New.

      New mail
    2. In the To field, start typing the recipient's name or email address until the recipient appears in the drop-down list.

      Outlook Web App Auto-Complete List
    3. Use the Down Arrow and Up Arrow keys on your keyboard to select the recipient, and then press Delete.

    4. Close or discard the new message without saving changes.

    5. If the NDR message has a Send Again option under the Report tab in Outlook or To send this message again, click here in Outlook Web App available, then click the link to open the original message to resend. If the send again feature isn’t available then create a new message.

      Send this message again link in Outlook Web App
    6. In the To box type the full address of the person you’re resending the message to. If it’s a new message then add your text to the message. You can copy and paste it from the original that you should find in your Sent folder.

    7. Choose Send to resend your message.

  • Solution 3: Ask the person you sent the original message to fix forwarding rules.    Did you send the original message but the NDR shows it wasn’t delivered to an email address you didn’t send to? It’s possible the person you sent it to has a forwarding rule set up that tried forward your message to a different email account that they also own, one where the email address doesn’t work. Contact the person you sent the original message to and ask them if they have a forwarding rule or delegate set up for their mailbox and ask if they know it's working correctly. If the person does have a forwarding rule, they will have to either correct the email address it’s forwarding messages to, or they should remove the rule.

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Administrator symbol

I'm the email admin. What can I do to fix NDR 5.1.10 issues?   

If the steps above don't solve the issue for the sender, the problem may be related to the way the recipient’s Office 365 account, domain, or on-premises mail system if it’s a hybrid topology, is set up. The Office 365 portal provides several things you can use to troubleshoot this. Log in to an administrator account on the Office 365 portal at and try one or more of following solutions:

Solution 1: Check for Mail flow rules that could be trying to forward the sender’s message to an address that doesn't work.   

  1. In the Exchange admin center, go to mail flow > rules.

  2. Look for any Redirect. . . rules that may be forwarding the sender’s message to another address. Example shown below.

    Exchange online transport rule with a redirect action

Solution 2: Double-check that the recipient address actually exists in your Office 365 organization and that an active license is assigned to it.   

  • In the Office 365 admin center choose Users > Active Users.

Solution 3: Check if there’s a current service issue in Office 365 affecting the recipient’s account.   

  1. In the Office 365 admin center choose Service health to see an overview of any issues for Exchange Online.

  2. Select the Information icon Additional information for any current Exchange Online issues that may be affecting the recipient’s account.

Solution 4: Check the Office 365Message center to see if your organization has a known configuration issue.   

  1. In the Office 365 admin center choose Message center.

  2. Select Prevent or fix issues and perform any relevant suggested actions.

    Office 365 message center list of issues to prevent or fix

Solution 5: Verify your domain's MX record.    If you have a custom domain (like instead of, and if the sender is outside of your organization, then there could be a problem with your domain’s Mail Exchange (MX) record. The MX record is copied across the Internet where mail servers can find it, and it tells them where to send messages destined for your domain. Your domain’s MX record should point to the Office 365<domain> fully qualified domain name (FQDN). Check for any incorrect or stale MX records using the Microsoft Connectivity Analyzer.

  1. In the Office 365 admin center, go to Tools > checks and select Check your Office or Office 365 connectivity with the Microsoft Connectivity Analyzer.

    Office 365 Admin center connectivity check
  2. Check with your domain registrar or Domain Name System (DNS) hosting service to verify that the MX record for your domain is correct. The MX record for a domain that's enrolled in Exchange Online uses the syntax <domain>

  3. Verify you have only one MX record configured for your domain. Microsoft doesn’t support using more than one MX record for a domain that's enrolled in Exchange Online.

    Note: Updates to a domain's DNS records can take up to 72 hours to propagate to all Domain Name System servers on the Internet.

Solution 6: Perform additional in-depth checks on your account by running the Office 365 health, readiness, and connectivity checks tool.   

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Why you got a non-delivery report with error code 5.1.10   

You'd see a notification message in your email inbox with status code 550 5.1.10 when you try to send an email message to someone whose email service is hosted by Office 365 but their address wasn’t found. The notification message about this issue is also referred to as a non-delivery report (NDR) or delivery status notification (DSN). To get an overview of NDRs, see What are NDRs in Exchange Online and Office 365.

If you didn’t actually send the original message, the non-delivery report could be backscatter and you can safely ignore it. Also, in rare cases, status code 5.1.10 is sent because the person’s email system isn’t set up correctly.

Still need help with error code 5.1.10?

Get help from the Office 365 community forums Admins: Sign in and create a service request Admins: Call Support

For more information

Create DNS records at any DNS hosting provider for Office 365

Customize an SPF record to validate outbound email sent from your domain

Configure email forwarding for a mailbox

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