Securing from data leakage through wrong recipients
One of the common data leaks sources is sending mail to people from different organizations by accident. It also includes sending mail to multiple people using To/CC instead of BCC as it reveals the e-mail addresses to unauthorized people.
It would be good to have a feature in EXO that can help address that, for example by detecting people from different orgs in one mail and informing about it using policy tips and blocking it with transport rule. Some kind of extension of DLP rules but not focused on contents but recipients.
I want to be warned that I'm sending to people from different companies in one mail, as this is something unusual. I've seen this multiple times that you want to send mail to firstname.lastname@example.org but you send to email@example.com because outlook has suggested the other e-mail and you did not check or just clicked the wrong one. Other case is that people send mass mailing and use the CC field instead of BCC and reveal e-mail addresses to all recipients which is data protection violation.
If Exchange could detect that there are recipients from multiple different domains and warn me about this fact then it could be easier to spot and stop such behavior. Also, having ability to have transport rule to detect that would provide second line of defense.