I finally upgraded my day-2-day notebook to Yosemite 10.10.2 and quickly noticed a very annoying problem with Apple's Mail client. Mail automatically saves drafts of emails as you compose them, which is okay and to be somewhat expected. However, the problem was, it would not delete the draft(s) after I sent the actual message and I had already turned off the "Store draft emails on server" function, years ago.
The solution I found that worked for me was to turn off the new "Automatically detect and maintain account settings" feature located in Preferences, Accounts, Account Name, Advanced, as shown in the screenshot below. After unchecking that check box for all of my accounts and their corresponding SMTP servers, I restarted Mail and everything now works as expected.