It's been worse since Yosemite.
From the same exchange account (office365) I will send and receive emails instantly on my phone, but they will take several minutes through the app.
Loading existing conversations also often takes a long time.
Conversations often get jumbled up and emails that don't belong will be added to a conversation.
And finally accepting event invitations is a little messed up. I click accept and nothing happens. Then a few minutes later the email will dissapear and the event will appear in the calendar.
I am having these issues on both my laptops.
I've tried offlining and enabling the account. Also tried reindexing.
Does anyone know if these are just Yosemite bugs? Or is there somethinng I can do to fix it?