One long standing issue with OS X 10.9 Mavericks and OS X 10.10 Yosemite is that it’s version of Apple Mail can be problematic when setup with multiple accounts, especially when Gmail or iCloud accounts are in the mix, even more so when there is a large amount of stored email or custom rules involved.
It peaked my interested when I read that the OS X 10.10.5 updates “improves compatibility with certain email servers when using Mail”. Thinking Apple may have finally resolved this issue, I researched this thoroughly in the Apple Support Community boards. There are a slew of recently posted horror stories about Mail problems after updating to OS X 10.10.5, so unfortunately I’d say things are about the same as before. https://www.google.com/search?client=safari&rls=en&q=10.10.5+mail&ie=UTF-8&oe=UTF-8#q=10.10.5+mail+site:discussions.apple.com
Unless you have just one email account, I would not recommend Apple Mail be used with either Mavericks or Yosemite. In my opinion, either Outlook 2011 or Outlook 2016 are better choices for anything beyond the basics.