Emails not sending, POST MASTER?

The error means the message is being blocked as spam by the mail system, not that sending is generally unavailable. Since it happens from both Outlook on the web and an iPhone, the issue is likely with the account or message content rather than the device.

Try these steps:

1. Send a very simple test message to a single recipient.
   • Use plain text only.
   • Remove all links.
   • Remove any signature.
   • Do not attach files.
2. If that works, add content back gradually.
   • First add normal body text.
   • Then add the signature.
   • Then add links or attachments one at a time. This helps identify what is triggering the spam block.
3. If sending to a group, reduce the recipient count and test with one person first. Group-style messages are more likely to be flagged.
4. If composing in Outlook on the web, switch the message format to plain text as a troubleshooting step.
   • Open Settings
   • Go to Mail > Compose and Reply
   • Change the message format to Plain text
5. If a postmaster message is returned, review the diagnostic section in that message. Look for a section such as Diagnostic information for administrators. It usually contains the reason the message was rejected.
6. If the message includes a large attachment, reduce it. Messages with attachments larger than 10 MB can fail delivery. Save the file to OneDrive and share a link instead.

A legitimate message from ******@outlook.com, [email protected], or service.microsoft.com usually means the message could not be delivered to one or more recipients.

• Failed delivery messages from the postmaster at outlook.com, microsoft.com, or service.microsoft.com | Microsoft Support
• Failed delivery messages from the postmaster at outlook.com, microsoft.com, or service.microsoft.com | Microsoft Support
• emails undeliverable - Microsoft Q&A Remote server returned '554 5.7.0 < #5.7.520 smtp;550 5.7.520 Message blocked because it contains content identified as spam. AS(4810)>'
• I can’t send emails. All are marked as spam - Microsoft Q&A
• Remote server returned '554 5.7.0 < #5.7.520 smtp;550 5.7.520 Message blocked because it contains content identified as spam. AS(4810)>' - Microsoft Q&A