Sync of send messages from iCloud to Mail blocks

Question

Hi all,
Since Apple had some problems with their iCloud service on June 18th, I experience very annoying problems with Mail.
When trying to sync mail with my iCloud account the "Send mail" box does sync everything properly, but the strange thing that happens every time is that the sync process as shown in the activity windows stucks. The progress bar remains halfway, the CPU usage of mail goes up to 200% and the fan goes crazy. I then can only "Force quit" the Mail program. 
When looking at the "Send" box all send mail is shown, but only the headers, not part of the content.
When clicking on a message the contents shows up properly in the separate mail windows.
(sorry fot the Dutch text :-)
So everything syncs properly but it looks like a specific process hangs.
I've tried several things:
Disabling Mail in iCloud system preferences
Deleting my iCloud account in Mail
Rebuilding all in- and out mailboxes
Rebuilding only the "send mailbox"
Restoring the complete Mail directory in the Library with a (Time Machine) backup from an earlier date
Deleting info.plist in the Mail directory
Creating another user on my Mac and initialize my iCloud account in Mail again for that user
Nothing helps.
So now every time I start Mail the normal steps are processed (as seen in the activity windows) but after a while it all ends up again as shown in the first image. (the progress bar pops up immediately halfway).
And I can only for a very short time check my mail, send messages before the CPU usage goes up again and the fan accelerates to a very high speed.
The only thing I can do then is "Force quit" the mail program.
Any idea what causes this?
Thanks for the help!
Gert

Answer 1

There are ways to get Thunderbird to use the correct Sent, Trash, Drafts folders for iCloud mail. You'll need to google how to do that. Not super straightforward, but it works. Note that unless you correctly map these folders, the mail you send using Thunderbird will not appear in Sent items on other computers, I don't believe. It will remain local to the machine that sent it.
Anyway, there are a few other things you could try to fix Mail. I was about to do this myself, but my problem seems to have gone away.
Open System Preferences and create a new user account with Admin priveledges (i.e. call it Test or something). Close System Preferences
While still in your account, go to ~/Library and copy your entire Mail folder to /Users/Shared (make a copy; don't move it).
Log out of your account and log into the new account you just created.
Browse to the Mail folder in Shared and copy it to ~/Library on the Test account. If you have never launched Mail before on that account, I don't think there will be a Mail folder in that location already
From this point, since you are working from a COPY, you can start experiementing. Start Mail and see if it works normally (if it does, then there is probably something wrong in your Prefs in your account). Try deleting some of the mail folders and starting Mail again to see if one of them was causing a problem, etc.
I don't have any specific tips related to step 5 -- I was just planning on experiementing to see what was causing Mail to hang. And since this is a COPY of your mail, it should be completely risk-free.
Good luck and let me know if you make any progress.