This article describes how to block a message from being sent or received in Microsoft Office 365 based on the file name extension of the attachment.
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 Office 365. To do this, create a transport rule by using one of the following methods, as appropriate for your situation.
Note 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 Office 365, follow these steps:
In the left navigation pane, click mail flow, and then click rules.
Click New (), and then click Create a new rule.
In the New Rule window, click More options.
In the Name box, specify a name for the new rule.
Click the *Apply this rule if drop-down list, point to Any attachment, and then click 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 click the plus symbol () to add the file name extension to the list. When the list is completed, click OK.
Click the *Do the following drop-down list, point to Block the message, and then click Reject the message and include an explanation or click 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 click OK.
Specify any additional options such as rule auditing and rule activation or deactivation time, and then click Save.