I ran into a somewhat annoying issue with using Mail Flow Rules with Exchange Online to modify the subject header on emails that were sent to aliases on some of my Office 365 accounts. I wanted to modify the email subject of those emails to show what alias it was meant for, and I figured I should share that process here if someone else runs into the same issue. This isn’t something that I do very often, and I was doing it wrong from the beginning, hence why I was having an issue.