Hi,
if you want to limit your users per day basis you need to work it out whit your users,… Important about this limit is accounting. Each message is acounted, so if you users are sending mails using multiple to,cc,bcc each of recipient is 1 message. So let’s say your limit is 100/day, your users always send mail to 5 recipient, than what seems to be reasonable limit of 100 mails is just 25.
So at least inform your users about your restriction. If your user base use Outlook and they hit this restriction, they usually does not see this, Outlook does not tell that message can not be sent.
Any way, even if you limit your users reasonable, it’s a good to make your self some kind of analytics/alerting. Unfortunately MC does not provide this capability. But you can forward all of your logs to external system and make some analytics/alerting there. I personally forward all logs to Loki and I got some dashboards in Grafana including alerting.
Also recently there was interesting third-party project tailored for MC (forum, git). Not sure if this project do what you need, but you can try to ask or make PR