The current version is abysmal but, in Apple's defence, I never had one problem with it in the 11 years I had my eMac. I've spent an hour on the phone with Apple Support today and, just as it looked as if they'd sorted all my problems, it's as bad as ever. What I am annoyed about is Apple's head-in-the-sand attitude to the problems in mail. They are in complete denial that there is a problem with the current version of mail and have insisted that the problems I've encountered can be fixed.
For the record, my own experiences include the following:
Slow downloading and delivery of messages.
Sent messages not appearing in the sent folder.
Phantom messages, i.e., the Activity bar showing that a message is downloading but none appears in any folder (or the trash).
Messages reappearing in my draft folder up to an hour after they were sent.
Mail application taking two or three minutes to close when I quit out of it.
What is really frustrating is that these problems are intermittent and so each time I call Apple Support the advisor thinks he's sorted the problem only for it to reappear again almost the minute I hang up. As a Mac user for over 20 years I expect better from Apple. We pay a premium to use Macs so the least we should expect is that they run smoothly.