when i hide mail it will periodically open by itself

Mail's red "close" button behavior has not changed. Certain macOS apps (like Contacts) will quit if you close all their windowed processes, while others (like Mail) will remain open even if you close all its windowed processes. In other words if you close all Mail windows without explicitly quitting Mail, it's still running. To get the Mail app to quit use Mail's File menu > Quit Mail.

I understand you're experiencing new behavior though. Try removing all Google accounts and services from your Mac as in that other Discussion, and see what happens. If Mail stops opening on its own then you effectively isolated the cause to Google, in which case you should contact them to determine what, if anything, you can do about it: Contact a third-party vendor - Apple Support.

FWIW, clicking on the red button does not hide/minimize Mail. It closes the Mail window. If you want to hide/minimize Mail click on the yellow button or press "Command +H" when Mail is in focus.

I understood your question. The red button does not quit Mail, it just closes the Mail window so the dot is still there. Just try the yellow button instead.

Well, I tested Mail for 6+ hours and it's still minimized. The only way I can get the window to open on its own is if I actually close the window with the "red" button which is what I suspected.

Hello bigstroke and welcome to Apple Support Communities.

It looks like no one has responded to your question. Here are a couple things that might help others answer your question:

When you say the app is "hidden" do you mean that it is not open or that it is minimized and sent to the Dock by using the yellow button in the upper left of the Mail app window?

When you say it opens periodically, does this happen when a new message comes in?

Any additional information you can provide may help the community to better answer your question.

Cheers.

bigstroke wrote:

I use Apple Mail with various accounts, ig: gmail, hotmail, yahoo, etc.

Read Ever since 10.14.5 mail app has been opening on its own (usually about half an hour or so after receiving new mail)..

Whose email service provider(s) are you using?

Either Mail is open, open in the Dock, or it is closed. Those are the options. The "dot" under an icon indicates an app is running.

I have three Gmail accounts and I can't reproduce the problem at all. I checked three of my Macs. So I don't know what the issue might be unless it's people using G Suite accounts. Beats me.

bigstroke wrote:

Now, it does not. My question is why?

You would have to ask Apple and they are not here. This is a users forum.