I agree with Sechs. After all, there is only so much you can desire from an email client. I've only used about 4 different ones ever. (Meaning "used" as in "installed on my own machine and used regularly for more than a day or two".) Let me see ....
Whatever the thing that came out of the box with OS/2 Warp 3.0 was. It was pretty dreadful. Mind you, at least you got something out of the box — the Windows world was still years away from having email at the time.
Then I used the excellent Post Road Mailer for years (another OS/2 native one).
Meanwhile on Windows boxen, I had the default Windows 95 client which was crap, soon followed by Netscape Messenger, which was (for the time) excellent. That remained my normal Windows email for ages.
But I switched over to PMMail on the OS/2 boxes, which was great because, by deliberate design, it does not render HTML emails. This was around the time that junk mail was becoming a major problem. Lack of HTML was a real help. (HTML sanitising Thunderbird style was still a long way off.) After a few years I got tired of having inferior products on my Windows systems so I bought a Windows licence for PMMail too. In both cases I used Junkspy to keep the spam under control.
Then Thunderbird arrived, and I found myself using it on Windows. The junkmail filtering is, once you train it a bit, even better than Junkspy. Just a couple of months ago I switched over to Thunderbird on OS/2 as well. Just one problem with it: it shares a non-sharable DLL with Mozilla, so you can't run them both at the same time. Well, you can, but you have to fiddle about adjusting your LIBPATH and I haven't got around to doing it yet.
Euadora has a bad habit of locking random machines up as soon as you install it. Or maybe it just freezes the app when you launch - I forget. Either way, I found that unacceptable, and almost never touch it. But they have probably fixed it now.
But in the end, email is email, and there is not a great deal of difference between any of the half-decent ones.