I agree that this problem needs to be fixed. Apple -- please pay attention to this one! We need a setting to limit the # of connections in IMAP, like other mail programs have.
I collected about half a dozen past threads in this forum about this problem going back from 2013 to 2006 (!!) at the following thread, and that was just scratching the surface -- there are tons of past threads about this on this forum, not to mention on every other forum on the Internet that discusses Apple Mail:
https://discussions.apple.com/thread/3797336?start=0&tstart=0