You gotta spoof the from email address. If you find an open mail relay, it's as easy as using the send-mail command in PowerShell. I never actually did that particular kind of operation in Linux, but I would be surprised if there isn't a cli that would do that.
Could be anything from on entry point on a phishing campaign to simply flooding their inbox with useless shit
Edit
Also, they likely use exchange, so if you were to spoof their email@ domain.onmicrosoft.com address, you could really trick people into thinking it was an official email from any of those addresses
The feeling of rust on my salad fingers is almost orgasmic