so I was on the phone to AppleCare on another matter... and I thought I'd get this one looked at... and SOLVED! 🙂
I was initially told by first support that they hadn't heard of this issue, so I pointed out this wasn't something new to Yosemite and has gone back years in OSX mail in my experience. A supervisor was put on the phone.
we started with the smaller of two Gmail IMAP accounts:
BACKUP BACKUP BACKUP <<< don't say I didn't warn you!
tried the un-enable/re-enable again... to no avail
then we opted to un-enable and add the Account again using the Mail/Preferences pane (didn't resolve the issue).
Then mail did something a little odd. On deletion of this replicated Gmail IMAP account it removed BOTH copies of that account! 😮
But when we re-added it again, it had resolved the Alphabetical mailbox issue!!! HUZZAH 😁
...due to the amount of mail that was then re-caching we opted to resolve the iCloud mail account next.
This time on clicking remove it launched the Internet Accounts pane Mail/ACCOUNTS not the Mail/Preferences/Accounts pane that we where in.
clicked on iCLOUD and unticked Mail. Back intoMail/Preferences/Accountsand deletediCloud account...this took us BACK to theInternet Accounts panewhere we opted to remove the entireiCloud Account.
** at this point you will be presented with lots of DELETE/ MERGE pop ups as you are removing contacts/calendars etc... so be warned!!!!
but on re-adding the iCloud Internet Account, it had resolved the Alphabetical mailbox issue again! HUZZAH 😁
Now the reason I've posted all the waffle to this simple REMOVE and RE-ADD solution
is that it didn't do what I was expecting when clicking the remove mail account symbol, and I thought it best to point this out to others having the issue.
and apart from it messing up my Address Book ON MY MAC Vs iCloud contacts (which was resolved thanks to that backup I took before I did all this)
all is now well in the Alphabetical mailbox dept (for me)
Hope it works for you too!