Error message on all outgoing emails- Message Blocked AS (4810)

For 550 5.7.520 / AS(4810), the message is being identified as spam by Microsoft filtering. Since it happens on all outgoing messages, including simple tests, focus on account and message checks rather than the recipient.

1. Send a plain-text test message from Outlook on the web with no attachment, no links, and no signature.
   If that works, add content back gradually to find the trigger.
2. Review message content for common spam triggers.
   Messages are more likely to be flagged when they contain:
   • excessive links
   • URL shorteners
   • embedded scripts
   • image-only content
   • complex HTML formatting
3. In Outlook.com, review mail settings that could affect sending or handling:
   • Settings > Mail > Junk email
   • Check blocked senders/domains and any unusual entries
4. If attachments are involved, test without them first. Some reports indicate attachments, especially combined with HTML formatting, can increase the chance of spam filtering.
5. Verify the Microsoft account status at the account portal to make sure identity verification or account checks are complete.
6. If simple plain-text messages with no links or attachments still fail, contact Outlook.com support from within Outlook.com:
   • Sign in to Outlook.com
   • Select Help on the menu bar and enter the issue
   • If self-help does not resolve it, go to Still need help? and select Yes
   • If Help does not open, sign in, then select the ? at the top of the Outlook.com window

• Frequently asked questions - Anti-spam protection
• 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 any emails containing at attachment. How can I fix this? - Microsoft Q&A
• Filter junk email and spam in Outlook
• Fixes or workarounds for recent issues on Outlook.com