Hey folks, I assisted a friend today who belongs to another business regarding Office 365 mail and his on premise firewall. He had issues whereby his exchange was used to spam but there are 2 reasons that this happened:
My recommendations and solution was for him to immediately turn off open relay on the server and to whitelist only Microsoft office 365 email servers to communicate to the firewall on port 25.
I used the article below which was useful and only used the portion relevant for his scenario so popped it in here for your perusal.
If you have a similar setup you can take a read on the article and implement if neccesary.
