exhange online non delivery report help

Question

Dear All!
We are using exhange online and some external domain is trying to send us a mail, and it gets refused with the following NDR:

ourdomain.mail.protection.outlook.com[104.47.x.x] said: 550 5.7.1
Service unavailable, Client host [193.169.x.x] blocked using Customer
Block list AS(1420) [DB5EUR03FT004.eop-EUR03.prod.protection.outlook.com]
(in reply to RCPT TO command)

The sent email in our tracking log do not appear. It is not in quarantine. Why should...we send back an ndr, but i am out of ideas... In EXO security center checked the antispam and phishing settings but i did not found relevant settings.
How sould I resolve this problem, what should i check?
Do you need any more information about the case?
Thanks
Peter

Answer 1

Thank you for your answer.
I did check this setting. The sender domain is not in the Block lists. In the spam and bulk actions we do not use the delete action. The international spam settings are off.
Adding the sender IP and to the allow list should resolve the problem, but I would like to understand the bigger picture. Also if the block is with good reason, and the sender is spaming, and if I add his IP to whitelist, that would be unfortunate.
Why, and where is this sender blocked?

Answer 2

@petersonal

I think the external domain may does not meet EOP requirements or triggers the sending rate limit, so their domain was added into Office 365 blacklist. You could confirm with Office 365 team, whether they could help you remove your panther domain from blacklist.

For now, you need to add your panther domain into safe sender list.

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Answer 3

Yes. Finally we agreed to whitelist the sender domain. It did not help. We got the same error. Today we did some testing again. I rechecked all the settings and found in the connection filter, that the sender email server address was blacklisted. I did check this before.... removed the IP now waiting for the result.