K9 and the forks and family are oldies and goodies. They are showing their age, but still, one cannot go wrong with them.
GMail and Inbox are resource hogs (along with most gapps). Microsoft's Outlook (not the Windows Outlook or Express!) is a decent lightweight contender and integrates calendar and contacts functions as well. This program is an acquisition. Its calendar functions will be incorporating those of another of their acquisitions, Sunrise calendar, which was one of the best. (Note that it plays very well with GMail accounts but it never read my own IMAP server through which I ran my pop3 accounts. I run the old pop3 stuff through GMail now so this works out just fine. Do not need the GMail app to do this--set it up on the site.)