Another client to recommend, largely as a “drop-in” replacement for Mail, is Postbox:
By drop-in, I mean one that still lets you use Apple’s Contacts and Calendar apps, rather than forcing you into a third party ecosystem. It also supports local “On My Mac” folders, and does not use third party servers, which can be a privacy risk.
Some apps are IMAP only, which means no local storage, and some will use their own servers to support functionality. In the latter case, they may use the content of your e-mail (possibly anonymized) to generate revenue from third parties for market research. So be careful if that is the case, especially if you use the app for work e-mail.
I moved to Postbox after giving up on Apple ever fixing the bugs in Mail that are the topic of a lengthy discussion here: