delivery failure message: Remote server returned '554 5.7.0

Question

Theo B 0

All my mails which I send via outlook via web are being returned with delivery failure message:

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)>'

Sello Makena 0 Reputation points

2026-05-04T09:25:17.7933333+00:00

try sending a plain text email (no links/images)
Theo B 0 Reputation points

2026-05-04T09:46:27.93+00:00

Already tried, that does not help.
Usario Usario 10 Reputation points

2026-05-04T10:03:12.2533333+00:00
Hey Theo, yeah that’s a super annoying issue 😅

From what you’re saying, it sounds like Outlook’s spam filters (or the receiving server) are flagging your emails as suspicious even though you’re sending normally. I faced something similar a while back and couldn’t figure out why legit mails were bouncing.

What worked for me:

Check your email content — even small things like too many links, caps, or certain words can trigger spam filters

Try sending a super simple test email (just plain text) to see if it still fails

Also worth checking if your account/IP got flagged somehow, especially if you sent bulk mails recently

Are all recipients affected or just specific domains?
Theo B 0 Reputation points

2026-05-04T13:50:34.4+00:00

Hi Usario,
I did send a simple message with below text in subject and mail header to 3 different mail addresses.

Test

All 3 came through, but if I reply to the mail it gets blocked again, a forward does not block it

According to Microsoft my account/IP is not flagged

Your answer

Sello Makena 0 Reputation points

2026-05-04T09:25:17.7933333+00:00

try sending a plain text email (no links/images)
Theo B 0 Reputation points

2026-05-04T09:46:27.93+00:00

Already tried, that does not help.
Usario Usario 10 Reputation points

2026-05-04T10:03:12.2533333+00:00

Hey Theo, yeah that’s a super annoying issue 😅

From what you’re saying, it sounds like Outlook’s spam filters (or the receiving server) are flagging your emails as suspicious even though you’re sending normally. I faced something similar a while back and couldn’t figure out why legit mails were bouncing.

What worked for me:

Check your email content — even small things like too many links, caps, or certain words can trigger spam filters

Try sending a super simple test email (just plain text) to see if it still fails

Also worth checking if your account/IP got flagged somehow, especially if you sent bulk mails recently

Are all recipients affected or just specific domains?
Theo B 0 Reputation points

2026-05-04T13:50:34.4+00:00

Hi Usario,
I did send a simple message with below text in subject and mail header to 3 different mail addresses.

Test

All 3 came through, but if I reply to the mail it gets blocked again, a forward does not block it

According to Microsoft my account/IP is not flagged