Mail window keeps opening without provocation

I'm having exactly the same problem since installing Sierra - skipped El Capitan where, from other folk's complaints, mail started having problems. FYI, if I minimize the mail window (yellow button in upper left of window when open) it doesn't open randomly.

I have the exact same problem with MacOS 10.12.5

The frequency of the problem seems to vary significantly as at times the window reappears within seconds of closing and this cycle repeats well over 6-7 times before I give up and quit the Mail app. Other times it will be open a day or two before having a fit.

I have tried the following remedies without improvement:

1. Quitting and restarting the app

2. Restarting the machine

3. Completely powering off and rebooting machine

Hardware: MBP 8,1 Late 2011

User settings for Mail are in the User Library>Containers folder:

~/Library/Containers/com.apple.mail

If you want to experiment: Quit Mail.

Move the com.apple.mail folder to the trash. Open Mail. You will need to recreate your settings. You can always reverse this if problems occur or to reset your original settings.

If the com.apple.mail folder doesn't do the trick, you could also repeat the process with the com.apple.MailServiceAgent folder.

If those don't work, then it may be something other than Mail causing the problem such as the Finder. But I wouldn't know what exact cause to speculate on.

I have had the same problem for some time. It seems longer, but it's probably been since 10.11.

It happens on every computer I have, my rMBP, and my work Macbook Air and iMac. The only thing I know is that it seems to happen when some type of authentication request times out. Like, when it is trying to connect to one of my accounts but runs into an error.

Actually saw someone else with the same problem, however I still have not other solutions to offer. Maybe if you could catch the exact moment it happens and look for log messages. That might tell you something.