I'm going to have to put a plug in for Google GSuite (gMail).  There are going to be those that say it can't be secure because its Google but its a widely used platform that is very successful. The bank I used to work for uses its and has no problems with it when it comes to regulators and auditors.  Remember, email is inherently INSECURE in that it a majority of the message is sent in plain text.  In my opinion Exchange is a dying platform and will only be kept around for large businesses with many users AND an experienced IT staff.  Exchange is (unless it has changed) somewhat difficult to manage.