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)

Remove any automatically inserted signature from a new plain-text message and test it with one recipient before escalating the persistent 5.7.520 AS(4810) rejection through Outlook.com’s in-app support.

Run one final controlled test

Plain text can still include a signature, links, or repeated boilerplate. Microsoft specifically identifies signatures as a possible cause of Outlook.com sending failures.^1^

1. In Outlook.com, select New mail.
2. Remove the entire signature if Outlook inserted one automatically.
3. Enter a new subject and one short sentence with:
   • no links,
   • no attachment,
   • no copied/repeated footer,
   • one recipient only.
4. Send it.
5. If successful, repeat with a small group. Do not immediately retry the 30- or 250-recipient message.

If the one-recipient message also returns 5.7.520 AS(4810), stop changing the message and escalate it as an apparent false-positive spam classification. Since plain text was already tested, there is no need to repeat other formatting tests.

Contact the correct support team

Microsoft 365 Basic is a consumer subscription using Outlook.com, so use Outlook.com support—not the Microsoft 365 business Admin Center:

1. Sign in to Outlook.com in a browser.
2. Select Help (?) on the menu bar.
3. Enter 550 5.7.520 AS(4810.
4. Complete the suggested troubleshooting.
5. Scroll to Still need help?, select Yes, and submit the request.^2^

Include:

• The complete error: 550 5.7.520 Message blocked because it contains content identified as spam. AS(4810).
• The failure from the new, signature-free, one-recipient test, if it fails.
• That plain-text messages without attachments are affected.
• That failures occurred with approximately 30 recipients, with a separate example involving 250 recipients.
• That the problem existed before purchasing Microsoft 365 Basic and continued unchanged after subscribing.
• The date, time, recipient domain, and full diagnostic text from at least one non-delivery report (NDR).
• The saved .eml or .msg NDR if the support form accepts attachments; otherwise paste its complete diagnostic details and message headers.

This is not the documented 250-recipient limit

For Microsoft 365 subscribers, Outlook.com publishes these maximums:

• 500 recipients per message
• 5,000 recipients per day
• 1,000 non-relationship recipients per day

Limits can still be lower depending on usage history, and newly created Outlook.com accounts can have temporary lower quotas while credibility is established.^3^

Therefore, 250 recipients is below the published per-message maximum, but that does not guarantee delivery. The AS(4810) response explicitly reports a spam classification rather than a normal quota-exceeded condition. Group size and sending pattern may still contribute to filtering, especially when many recipients have no established correspondence with the sender.

While support investigates:

• Avoid repeatedly resending the 30- or 250-recipient message. Microsoft recommends reducing sending frequency or volume when deliverability is impaired.^4^
• Add regular recipients to Outlook.com Contacts and communicate with them individually where practical.
• Use smaller batches only as a temporary measure, not as a guaranteed bypass.
• Do not use the IP delisting portal unless mail is sent through a server or custom domain under your control; it is not the appropriate route for a normal Outlook.com mailbox.

References

1. Can't send or receive email in Outlook.com | Microsoft Support
2. Sending limits in Outlook.com | Microsoft Support
3. Sending limits in Outlook.com | Microsoft Support
4. Sender Support in Outlook.com - support.microsoft.com