My problems started with OS X 10.9
Never really posted before, but need to for this one….
I can relate to the issue of having a number of accounts and having issues with Mac Mail.
Yes, while millions of people have no issues with Mail, I would propose because their needs are easy.
But what about us who try to incorporate Macs for business use and push the program to the edge? Doesn't Apple want business users to be using this program too?
I own a small business of 30 people and have 75+ Macs I own and push Mail to the limit using it to check email for 20+ email addresses. For years, I pushed my team members to use Mac Mail because of how great it was. Luckily for them, they do not have to check so many email accounts.
Up until OS X 10.9, I never had an issue ( except Version 4 issue shown here -
http://en.wikipedia.org/wiki/Mail_(application) ).
I loved it for the ability to search for emails 5 years old in seconds.
Once I upgraded one of my Mac Pros to 10.9, the issues started. I went to SSDs, I upped my RAM, and still the issues are so bad, that even bought a new $5K Mac Pro (running 10.10) and that has Mail lagging issues. Now, I bring my MacBook Pro running 10.8 in JUST to check my mail and do the rest of my work on my Mac Pro with 4 monitors.
Even a Mac Pro I keep in a remote office running Snow Leopard still does great checking all my mail accounts as does my iPad Air, iPad Mini and iPhone 6+, but not any computer I have with 10.9 or higher.
I do not know what happened, but something is definitely wrong and it is not an email setting, I can assure you.
I am currently looking to by a refurb'd high end 27" iMac that has 10.8 to replace my Mac Pro and fix my issue at the office of spending 3 hours on email.
While I know many people say - the issue is you are checking too many email accounts - my response is, why does 10.8 or older work without issues?
I blame Apple for getting me hooked on Mail and then pulling the rug from under me making me fall. I have already put 40 hours in trying to fix this…
And here is some good info -
Email tips for Yosemite, use Mail's new features - How to - Macworld UK