How to block a message from being sent or received based on the file name extension of the attachment in Microsoft 365
Original KB number: 2795329
You can block a message from being sent or received if the file that's attached to the message has an extension that's currently not blocked by Microsoft 365. To do this, create a transport rule by using one of the following methods, as appropriate for your situation.
The rule doesn't apply to scenarios in which users change the file name extension before they send the message.
To create a transport rule to block messages based on the file name extension of the attachment in Microsoft 365, follow these steps:
Sign in to the Microsoft 365 portal.
Select Admin, and then select Exchange.
In the left navigation pane, select mail flow, and then select rules.
Select New ( ), and then select Create a new rule.
In the New Rule window, select More options.
In the Name box, specify a name for the new rule.
Select the *Apply this rule if drop-down list, point to Any attachment, and then select File extension includes these words.
In the Specify words or phrases window, type the file name extension of any attachment that you want to block, and then select the plus symbol ( ) to add the file name extension to the list. When the list is completed, select OK.
Select the Do the following drop-down list, point to Block the message, and then select Reject the message and include an explanation or select Delete the message without notifying anyone.
If it's required, add a statement to inform users who will receive the non-delivery report (NDR) of the reason that mail delivery failed, and then select OK.
Specify any additional options such as rule auditing and rule activation or deactivation time, and then select Save.
Still need help? Go to Microsoft Community.