Apologies for not being sufficiently specific. I do double-click on the mail icon, without response.
This can't be the case unless you've created a copy of the application. Did you do that?2. It's the actual mail icon, not an alias
3. It's in the dock - apologies again for using imprecise language.
When I drag the mail icon onto the desktop it doesn't disappear - just jumps back to the dock.