How to fix error code 554 6.6.0 on my Outlook email account? I can't send or reply to emails. I receive emails in the inbox.

For an Outlook.com account that can receive mail but cannot send or reply, and the message is being flagged as spam, use these checks:

1. Send a simple test message from Outlook on the web.
   • Remove attachments, links, repeated text, and anything that could look spam-like.
   • Send plain text only.
   • If that works, add content back gradually to identify what triggers the block.
2. Check the sending address in Outlook on the web.
   • Go to Settings > Mail > Sync email.
   • Verify the From address is correct.
   • Then go to Settings > Mail > Compose and reply and reset the default From address.
3. Remove the email signature and try again.
   • Signatures can sometimes prevent sending.
4. Confirm the message size is not too large.
   • Outlook.com cannot send a message with an attachment larger than 25 MB.
5. Review account security.
   • If unusual sign-in activity was detected, the account might be temporarily blocked.
   • Check recent sign-in activity and, if needed, change the password.
   • If the password was changed recently, update it in Outlook.
6. If the message is still blocked, use the non-delivery report details with Microsoft support through Sender Support in Outlook.com.

Since incoming and outgoing settings are already correct, the most relevant next step is to test with a plain-text message in Outlook on the web and verify the From address configuration there.

References:

• emails undeliverable - Microsoft Q&A
• I can not send emails but I receive them - Microsoft Q&A
• Can't send or receive email in Outlook.com
• I can't send or receive messages in Outlook