To add to this topic, finding alternates to Thunderbird (my main email client, still) is a rampage I go on every few years. I add one primary requirement to any email client that I will consider using. It must use the industry standard MBOX format for its mailboxes and stored messages.
Why? Because then your email boxes can be moved seamlessly between your current MBOX-based client and any new MBOX-based client you wish to try out. To do otherwise is to lock up your email in someone's proprietary format, leaving you dependant on their import/export capabilities to move your email to any other client.
I always avoid proprietary formats for all my media. It keeps everything portable.
Back to the main topic, my "every few year" search for an alternate to Thunderbird usually leaves me disappointed and I carry on with Thunderbird again for another year or two. I have been doing this ever since Thunderbird was first released!
If/when I do find a client of interest, it almost always wants a monthly subscription fee these days, and with Thunderbird being free, fully operative and meeting all my email needs (I am just bored with it) another monthly fee is hard to justify.
