So…is that to say you've had issues with Mail.app in the past, and have tested it on El Cap and are still having issues?
Mail.app has been nothing but a complete disaster for me (my clients) since 10.9. (and 10.8 wasn't great). But in 10.9 and 10.10 it became nearly unusable. Stalls, no new mail notifications, PWODs, terrible performance, list goes on and on and on. IMAP, Google IMAP, iCloud, none of it mattered. It is not unusual to see threads in the Progress dialog crossing between events (which means the thing isn't thread safe!).
So I'm very interested in knowing if Apple has fixed Mail. Tim Cook promised Mail would be fixed in 10.10 at last year's WWDC, which meant they ACKNOWLEDGED the problem (for a change); it wasn't. The lack of acknowledgement of that in the 10.11 keynote was disconcerting. [It is so bad, users are talking of moving to Win 10 when it ships. They were switchers from XP, they WILL go back. Take note, Tim Cook!]
Mail needs a complete rewrite. Not the UI, but the inner workings of the app need to be rewritten. Ever tried to export messages from an iCloud account to Exchange? The way Mail stores messages is a big failure. Apart from that, Mail can be nice..
I gave up on Mail.app a long time ago and moved to MailMate. That app has worked for more than two years without a single issue for me until El Cap, where it started crashing. So, I had to use Mail for a bit now. Luckily the dev released a beta that works on El Cap just an hour ago and I can switch back.
With that aside, I did play with Mail.app a bit on El Cap with a small email account. It seems fine but it doesn't feel like anything changed for the better. I'll add a heavy one in a while and see if it is stable.