"There are plenty of good freeware email clients out there."
I'm afraid the number is instead close to zero. Even Thunderbird didn't go far, and it has an ill-thought GUI that is far from intuitive (i.e., why you have to drag a file to a small are of the screen to attach it???). Its account management is still a complex mess for most users.
Pegasus was nice years ago, but its author put itself in a dead end trying to reinvent and rewrite several wheels (and almost starving because of it). Pegasus version 5 was promised years ago. Now he's happy he wrote his own help system.... while he's still trying to use its own HTML rendering engine - good luck (you may not like HTML emails, but you still may need to read them...).
The Moldovan The Bat looks to be still alive, while I never used Mailbird. The former is not free, the latter has a free edition with some limitations. It doesn't look "plenty" - is there something I forgot which is still actively developed?