I’ve been bothered with this issue for several times. I can’t shut down my Mac because of Mail, an app I use daily.
It seems Mail won’t quit and it shows me this error message:
I remember I quit the Apple Mail app, but somehow it just failed…weird!
After attempting to research the reasons why, I also bumped into a few forum discussions where other Mac users were also experiencing the same issue.
And to my surprise, we can’t figure out the exact reasons. Some say it’s related to the OS X (now macOS) your Mac is running under, while there are also people say it’s due to the Mail app itself — meaning it’s a bug that needs to be fixed.
Personally, I feel the later sounds more reasonable. Because I’ve encountered a few other issues related to Mail, such as the app takes longer to receive and send out new mails. Especially when I send an email, sometimes it just freezes on the “sending” process, I often have to double check the sendbox to make sure my email reply is delivered successfully. Apple already knows this issue as you can see from this support article.
Another case, which is more recent — the Mail app has some problems after iPhone and iPad users updated to iOS 11. Apple and Microsoft ware working together to fix the problems, as you can read more from this MacRumor report.
Anyways, the solution to fix that Mail failing to quit error is also simple.
Simply click the Apple logo on the top left corner, then select Force Quit, highlight the Mail app, and confirm the operation. After that, your Mac should be able to shut down properly.
Another way to do this is open Activity Monitor (do a quick Spotlight search? to open the app), then select the Mail related process and quit it.
You can choose either way to get that done.
That’s for this update, I hope you find it useful. And hopefully Apple Mail team would improve the app and make it more smooth to use 🙂