Tag incoming mail came from other domain - mail flow rule

Create an Office 365 Transport Rule to modify the message subject to make it identifiable
Login to your Office 365 Admin portal and go to Exchange administration.
Go into the “Mail Flow” section.
Click the add button and select the option to create a new rule.
The new transport rule window will be displayed.
Give your transport rule an appropriate name
Click the “more options” link at the bottom of the window.
Under the “Apply this rule if…” option
The sender is located outside the organization
From the “Do the following” menu option choose “Prepend the subject of the message with…”
Enter the text that you would like prepend to the message subject line. e.g.
Add any additional conditions that you would like to apply and then click “ok”.
Save the transport rule.
Send an email to the address that you are trying to identify and you should see that the subject line

 Please refer screenshoot for your reference.