I'm currently using eM Client your email because it has one critical for me feature. it can make a single Google Calendar its primary calendar. I have two accounts, one on Google (primarily for calendar and drive), and the actual business account which is privately hosted.  I want the home / primary calendar of the privately hosted account to be the Google Calendar of the Google account period in other words, when I get invites on either account I want the invite added to the Google Calendar. I also have two other pairs of Google Google account and non Google account I want to treat the same way.