Can't receive outside emails

Question

How do I fix not receiving emails from external servers

Answer 1

Dear @Edith Lagos,

Thank you for posting your question in the Microsoft Q&A forum.

I understand how disruptive this can be, especially if you’re expecting important messages from outside your organization. When external emails are not being received, the issue is usually related to Exchange Online or tenant‑level settings.

To help us pinpoint why you’re not receiving emails from external senders, could you please confirm the details below?

• Are you unable to receive emails from all external senders, or only from specific domains or senders?
• Are other users in your organization able to receive external emails successfully?
• When did you first notice this issue, and was email from external senders working correctly before?

In the meantime, here are some steps I recommend you check again:

Step 1: Check via Outlook on the web

Please sign in Outlook for web and see if external emails arrive there.

• If they do not appear in Outlook on the web, the issue is server‑side (mail flow or tenant settings).
• If they appear on the web but not in the desktop app, the issue is likely local to Outlook.

Step 2: Mailbox delivery restrictions

An administrator should verify that your mailbox is not configured to block external senders. In particular, ensure that “Require that all senders are authenticated” is disabled, and that no external addresses or domains are listed under rejected senders. For more information, please follow the steps here: Use the EAC to place message delivery restrictions

Step 3: Domain and MX record configuration

If your organization uses a custom domain, please confirm that the MX record points correctly to Microsoft 365 and that the domain is fully set up. An incorrect or missing MX record will prevent external mail from reaching Microsoft entirely. For reference: Connect your domain by adding DNS records

Step 4: Sender feedback / bounce messages

If possible, ask an external sender whether they receive a bounce‑back (non‑delivery report) when emailing you. The error message can help identify whether the email is being blocked or rejected. For reference, please refer to Email non-delivery reports and SMTP errors in Exchange Online

Step 5: Spam, quarantine, or mail flow rules

If message trace shows emails as delivered but they are not visible, an admin should also check spam policies, quarantine, and mail flow rules that may be filtering external messages. For more information, please refer to:

• Configure anti-spam policies for cloud mailboxes
• Mail flow rules (transport rules) in Exchange Online
• Quarantine policies in cloud organizations

Note: Please understand that as a forum moderator, my role is to provide general guidance and troubleshooting support. I don’t have access to internal systems or backend tools, and I appreciate your understanding of these limits.

I hope the information shared helps point you in the right direction. Please try the steps above and let me know if they work. If not, we can continue working together to narrow this down.

The first response may not always resolve the issue right away, but with your help and more details, we can work toward a solution.

Thank you for your patience and understanding. I'm looking forward for your reply.

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Answer 2

For external senders not reaching a mailbox, focus on mail flow and recipient restrictions in Exchange/Outlook on the web.

1. Check if the mailbox or target object is restricted to internal senders only
   • In Exchange Online, a mailbox, public folder, or distribution list can be configured to accept mail only from authenticated (internal) senders.
   • If external senders get NDRs like 550 5.7.1 RESOLVER.RST.AuthRequired; authentication required or similar 5.7.1 errors, the recipient is likely restricted.
   • An email admin should:
     • Open the Exchange admin center (EAC).
     • For a user mailbox: go to Recipients → Mailboxes, select the mailbox → Mailbox features → Message delivery restrictions → View details.
       • Either clear Require that all senders are authenticated, or
       • Use Only senders in the following list and add the needed external senders plus the “all internal users” group.
     • For a public folder: go to Public folders → Public folders, select the folder → Edit → Mail flow settings → Message Delivery Restrictions.
       • Clear Require that all senders are authenticated and select All senders so external mail is accepted.
2. Check if the external sender’s IP is blocked by Microsoft 365
   • If external senders receive NDRs mentioning 5.7.1 Service unavailable; Client host [x.x.x.x] blocked using Blocklist or 5.7.1 Unable To Relay: Blocked by Customer Allow list, their sending IP may be on a blocklist.
   • An email admin should:
     • Review inbound connectors and the IP Allow list in the EAC connection filtering settings.
     • If the external IP is both on an inbound connector and on the IP Allow list, remove it from the IP Allow list so the NDR exposes the specific blocklist name.
     • Ask the external sender to delist their IP with the blocklist provider (for Microsoft’s own blocklist, the NDR typically instructs them to forward the message to the delist address shown).
3. Confirm the issue is only with external mail
   • Verify that internal mail (from colleagues in the same organization) is delivered successfully.
   • Ask at least one external sender to share the exact NDR text they receive; look for 5.7.x codes and phrases like authentication required, blocked using Blocklist, or Unable To Relay. This points directly to either recipient restrictions or IP reputation.

If not an admin, contact the Microsoft 365/Exchange admin with:

• The affected mailbox address.
• Sample external sender addresses.
• Full NDR text received by those senders.

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References:

• Fix NDR error "550 5.7.1" in Exchange Online
• 5.7.1 Unable to Relay: Blocked by Customer Allow List
• Fix NDR error "550 5.7.134" in Exchange Online