Transport rule to block emails with attachments.

To create an exchange transport rule to block messages with attachments in Microsoft 365 to the outside of your organization, follow these steps:

1. Sign in to the Microsoft 365 portal.
2. Select Admin, and then select Exchange.
3. In the left navigation pane, select mail flow, and then select rules.
4. Select New( ), and then select Create a new rule.
5. In the New Rule window, select More options.
6. In the Name box, specify a name for the new rule.
7. Select the *Apply this rule if the drop-down list, point to the recipient is located, and then select outside the organization.
8. Add another condition
9. Select any attachment and then from the dropdown menu select a size greater than or equal to 1 KB.
10. 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 is 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.
11. Add an exception the recipient is located and then select inside the organization. (optional)
12. Add an exception if you need to send an email to a specific id with an attachment. (optional)
1. Add an exception to the recipient and then add the specific email address.
13. Specify any additional options such as rule auditing and rule activation or deactivation time, and then select Save.

This rule will block all the emails sent outside of your organization with any type of attachments.

Note: If the email contains any type of picture in the signature or anywhere in the message body the rule will consider that image as an attachment and will block the email.