Summary

This guide helps you troubleshoot and resolve: Email delivery delays. Follow the steps below to fix the issue.

Common Causes

Overview

Emails taking too long to arrive? This guide helps diagnose and fix email delivery delays in Microsoft 365.

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Normal Delivery Times

• Internal emails: Usually instant (seconds to 1–2 minutes)
• External emails: Typically 1–5 minutes
• High-volume periods: May take longer during business hours

Warning signs: Emails taking 15+ minutes consistently.

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Quick Checks

1. Check Your Send/Receive

1. In Outlook, click the Send/Receive tab.
2. Select Send All to force delivery.
3. Verify that no messages are stuck in the Outbox.

2. Check the Outbox for Stuck Emails

1. Open the Outbox folder.
2. If messages are present, double-click each one and click Send, or resolve any error messages.

3. Verify Your Internet Connection

• Open a web page to confirm connectivity.
• If the connection is slow, email delivery may be delayed.

4. Review Attachment Size

The default Microsoft 365 email attachment limit is 25 MB. Files larger than this should be shared via OneDrive and sent as a link.

5. Check Service Health

Visit the Microsoft 365 Service health dashboard for any known mail-flow incidents that could affect delivery.

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Troubleshooting Steps

Force Account Sync

1. Click Send/Receive → Update Folder in Outlook, or press F9.

Repair Outlook Account

1. Go to File → Account Settings → Account Settings.
2. Select your email account and click Repair.
3. Follow the wizard to complete the repair.

Clear Outlook Cache

1. Close Outlook completely.
2. Press Win + R, type %localappdata%\Microsoft\Outlook\RoamCache and press Enter.
3. Delete all files in this folder.
4. Reopen Outlook.

Check Quarantine and Spam Filters

Inspect Transport Rules

Use Message Trace

1. Open the Microsoft 365 Admin Centre and navigate to Exchange admin centre → Mail flow → Message trace.
2. Filter by date, sender, and recipient to locate delayed messages.
3. Examine the trace details for any “Queued” or “Deferred” status.
4. If messages are stuck in a queue, review any transport rules or connector issues that may be causing bottlenecks.

Verify DNS and MX Records

1. Ensure your domain's DNS records are configured correctly.
2. Verify that MX records point to Microsoft 365 mail servers.
3. Check that there are no conflicting or outdated DNS entries affecting mail flow.

Check Authentication Failures (SPF, DKIM, DMARC)

1. Verify that your domain has valid SPF, DKIM, and DMARC records published in DNS.
2. Authentication failures can cause recipient servers to delay or reject incoming mail.
3. Review the message trace results for authentication-related delivery statuses.

Check Recipient Mailbox Quota and Server Blocks

1. Confirm that the recipient's mailbox is not full.
2. Check if the recipient's email server is blocking messages from your domain.
3. Ask the recipient to check their own spam or junk folders.

Greylisting Delays

Some recipient mail servers use greylisting, which temporarily rejects email from unknown senders. The sending server retries after a short delay, which can cause the first message from a new address to arrive later than expected.

Cached Exchange Mode and OST Corruption

If you use Outlook in Cached Exchange Mode, a corrupted Offline Storage Table (.ost) file can cause the appearance of delivery delays or missing messages. Try creating a new Outlook profile or switching to Online Mode temporarily to test if the issue persists.

PowerShell Queue Inspection (Admins)

Connect-ExchangeOnline -UserPrincipalName [email protected]
Get-Queue | Where-Object {$_.MessageCount -gt 0} | Format-Table Identity,MessageCount,NextHopDomain
Get-MessageTrace -Recipient "[email protected]" -StartDate (Get-Date).AddDays(-1)

Replace [email protected] with your actual tenant admin UPN.

Check Message Headers for External Delays

1. If emails are delayed externally, request the full message headers from the recipient.
2. Review the headers to identify where the delay occurred in the mail path.
3. If the message is delayed at the recipient's server, advise the recipient to contact their IT team for further investigation.

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Prevent Future Delays

• Avoid sending large attachments during peak Microsoft service periods.
• Keep attachment sizes under 25 MB or use OneDrive links.
• Maintain a stable internet connection.
• Regularly update Outlook and apply Microsoft 365 updates.
• Consider using Outlook on the web as a backup during outages.

Still Having Issues?