Apple mail opens untitled mail window

I had the same problem on Sonoma

That was caused by 2 emails stuck in outbox.

Once deleted issue is resolved

Hey there obewon, 

It sounds like Mail be opening a new message or some other window when you are first starging up your Mac. If so, you'll want to take a look at the guide below to prevent Mail from opening when you log in.

Prevent apps and windows from reopening on Mac   

"macOS Ventura or later

1. Choose Apple menu  > System Settings.
2. Click Desktop & Dock in the sidebar.
3. Scroll down on the right and turn on “Close windows when quitting an app.”

Earlier versions of macOS

1. Choose Apple menu  > System Preferences.
2. Click General.
3. Select ”Close windows when quitting an app.”"

You'll also want to follow the steps found under, "Other solutions"

Have a good one!