How to Block Emails By smtp.helo in Exchange

Question

Hello,

I want to block emails through exchange mail flow rules based on the smtp.helo domain because custom email domains hosted on proton or tutanota won't be blocked unless we block the proton/tutanota smtp.helo.

Basically, I need to block emails based on analyzing header text.

I don't see the option to do that in the mail flow rules:

https://learn.microsoft.com/en-us/exchange/security-and-compliance/mail-flow-rules/mail-flow-rules

Does anyone know how to do this?

Answer 1

Hi @StudentAdmin ,

Thank you for posting to Microsoft Community.

Based on your description, I understand that you want to create a mail flow rule to block email from a specific domain based on header text and you did not see the option with Message header.

You could refer to Mail flow rule conditions and exceptions (predicates) in Exchange Online | Microsoft Learn for more information about the Message headers condition.

Therefore, you could create the mail flow rule match your need in Exchange admin center (EAC)  as the following steps.

1. Sign in to the Exchange admin center (EAC).
2. In the left navigation pane, select Mail flow rules.
3. Click + and select Create a new rule.
4. On the New rule page, enter a name for the rule in the Name box.
5. In the Apply this rule if… section, select The sender is and then select Domain is.
6. And add the condition The message headers.
7. In the Do the following… section, select Block the message.
8. Click Save.

Hope it helps and if there are anything else you need help, please feel free to contact me.