my email window keeps flashing on and off, back and forth

Thanks for reaching out, wiota.

It sounds like you're noticing some unexpected behavior with the Mail app. We can help.

When you say that you're seeing the Mail app flash, do you mean that you're seeing some sort of alert? We're also not sure what you mean when you say back and forth and up and down... is the window itself moving on the screen?

Are you on the latest macOS? If not, let's try updating first and see if the problem persists: Update macOS on Mac

Keep us posted.

Start up a Mac with Apple silicon in safe mode

On your Mac, choose Apple menu  > Shut Down.

After your Mac shuts down, wait 10 seconds.

Press and hold the power button on your Mac until the startup options window appears showing available startup disks and Options.

Select a startup disk.

Press and hold the Shift key, click Continue in Safe Mode, then release the Shift key.

To leave safe mode, restart your Mac normally, without pressing and holding any keys during startup.

If still no good Disconnect from the Internet, then start Mail, look for the last rec'd eMail.

Does Mail crash if no internet?

Quit Mail & login to your WebMail in a Browser, if possible delete any later eMails there after the last rec'd one on the Mac.

If you have no POP accounts & they’re all IMAP accounts then in Mail’s Mailbox menu choose Rebuild.