apple mail getting an untitled window

Had the same problem on a 14" M1 MacBook Pro running macOS Ventura 13.1, and finally fixed it. Problem was being caused by ONE old unsent email in the outbox of one of my internet Mail accounts.

I was having blank, greyed out windows appeared over Mail that looked like an empty 'compose' mail window except that it was completely greyed out and I could not type in it or close it. I could resize it or minimize it, but it would never go away. The number of blank windows would vary - from 1-2 to 6-8 - without any clear reason why.

I tried all the usual fixes - force quit Mail; turned off my Mail internet accounts, rebooted, and turned them back on; rebuilt each internet account's mailbox; opened Mail w/ shift key pressed; deleted preferences files; ran laptop in safe mode; ran Disk Utility; even reinstalled Ventura OS - but nothing made the blank windows go away.

Called Apple Tech support and spent time with a senior tech diagnosing the problem and trying to fix it with no joy until the tech suggested we turn off accounts one at a time to see if that affected the number of blank windows (it did!) and then went looking into the outboxes of the internet mail accounts, especially the one that seemed to have the largest effect on the number of blank windows displayed. There was ONE old email (response to an event invitation) that had not been sent for some reason. Soon as we deleted that one outbox email and closed / opened Mail the problem was gone! The email had no attachments or large file size, was just a simple 'decline' to an event invitation that got stuck in the outbox.

I'm not yet certain the problem has been solved for good, but at least now I have a path to fix it without having to do a bunch of software reinstalls.

I hope this experience is helpful to others. I spent a lot of time trying to fix the problem and owe the solution to a diligent Apple tech support individual who kept looking until we found something just a little unusual that was causing the problem...