It seems as of 2017, users have begun reporting the annoyance of MacOS Mail randomly popping up in full screen mode. Many users have proposed fixes to this issue, however they have not found a solution that actually works. And that’s why I decided to write on a better proposed solution that actually works, with some caveats of course.
Many users have thought to have fixed their Macs annoying mail popup, but it has appeared that no fix has worked, for long. One solution was to change the Mail-apps settings, not to show in full window mode. A second solution involved minimizing the Mail-App instead of force/quitting the app (something to that degree). There was also the “ditch the app” as a last solution. But the one that received my full attention was the “remove notifications from gmail”. What is most particular is that all users agreed, gmail was involved – somehow – in this mess.
Personally I think it’s a flaw designed by both gmail and MacOS, incidentally, causing chaos. Personally, I have a hunch that MacOS mail app folders are being no longer synchronized with gmails label systems.
To skip my rambling, just scroll down to My Proposed Solution.
Conspiracy Time. Sure, gmail is “experimenting” to help filter junk mail by mark it with a label… instead of placing it into a folder.. But the reality of the situation is that Google and Apple have had incompatibility issues for years. And both giants most likely finding ways to make more money. Have you tried to swipe to delete an email? Although its logic, to keep important emails in the inbox, the innovative Apple giant has decided to replace the way one usually deletes emails with archiving. The bigger picture is all about storage.
In a capitalistic world, anything useful is highly marketable. In todays world, logic is used against users. Think about it, storage is generally shared by different services and that includes mail. In Apple’s world, less storage means a sh** load of notifications to upgrade services… The big difference with Google is more storage, more storage means that long time users will have to decide their options, maybe in years down the road.
My Proposed Cause to the Problem involves both Apple and Google. Typically when my mail app opens at random, there is a notification sound and my mailbox opens in full screen… but no mail appears in the inbox.
Typically a gmail server will send a notification (STMP) to a device, letting it know new there is mail. Here, mail means everything that is unfiltered. Apple’s MacOS internal mail process responds, giving the gmail server the “I’m online, ready to download” message, while simultaneously telling the internal mail app to download the message. And the email is filtered as junk, triggering the start of an ambiguous error.
But that’s just my proposed thoughts of what the problem is, without evidence and no ambition to followup on my claim.
My Proposed Solution

Take all accounts offline. A caveat. Apple’s Mail App has had a hidden option, possibly since snow leopard to take all mail accounts offline. And just as annoying, it stops the windows opening phenomena.

The take off all accounts icon can be dragged into the tool bar by right clicking or with the double finger click (trackpads). Select the customize toolbar option, find the icon and drag that sucker into the toolbar.
The magic starts when you push the button. And just as mentioned, there is a caveat. Now to send or receive mail, the icon has to be switched. Works well if the Mac mail app is shut off or minimized.