So we have one user who seems to be spamming many other users. I figure its something local to his machine, malware etc. I have adjusted spam settings for the domain but I was wondering if there is anything I can do to limit the amount of spam he is sending out from 365?
There is also third party solutions such as Mimecast which offer Internal Email protect between the users mailboxes which ought to cut down on a lot of it if you decide to go the third party route.
Because it could be a breach I would also consider some additional steps such as scanning of the machine with a reputable endpoint solution, activating credential and device guard if using Windows 10 and enrolling their devices into Intune in order to manage and protect their devices against modern threats.