Does Mail show up in the Applications folder, and can you launch it normally by double-clicking on it there? If yes, then I'd try the process outlined below. When reading, replace "Internet Explorer" with "Mail".
Caveat: I'm no Apple guru, do at your own risk, if it gives you herpes that's your problem, etc etc.
http://docs.info.apple.com/article.html?artnum=106469
Mac OS X 10.1: Question Mark Appears in Dock
A question mark may appear in the Dock where an application formerly appeared. This affects Microsoft Internet Explorer when upgrading to Mac OS X 10.1.
Symptom
The Dock may have one or more question mark (?) icons.
Products affected
Mac OS X 10.1
Microsoft Internet Explorer 5.1.2 (3707)
Solution
This occurs whenever an item in your Dock cannot be found. An item may not be found when:
An installer replaces an application with a different version in a new location.
You remove the item from your computer.
The item in the Dock was stored on a disk that is no longer present or available (mounted). This includes items dragged to the Dock from a previously-mounted disk image.
If the Applications folder is renamed or moved when started up from Mac OS 9. You should not rename or relocate Mac OS X-installed folders.
For example, the Mac OS X Installer replaces Internet Explorer with a new and improved version. If you had Internet Explorer in the Dock, you would follow these steps to delete the question mark and replace it with an icon of Internet Explorer:
1. Drag the question mark from the Dock to the desktop. It disappears.
2. Click the Finder icon in the Dock.
3. Choose Applications from the Go menu.
4. Drag the Internet Explorer icon to the Dock.
Edit: Oops, nostatic beat me to it and better too. I'll leave this post up, for the link.