Dock is not visible when Mail app is open.
How do I make the dock visible when I'm using my Mail app?
iMac 21.5″, macOS 12.6
I suppose that you have Mail open in full screen, otherwise I can't see how the appearance of the Dock would change.
You can always move your cursor to the Dock location, and it will show... but WHY?
That would defeat the purpose of using an application in full screen.
Of course, you can also take Mail out of fullscreen, by pressing Escape.