this is a copy paste from modmyi, post by DarkWombat
Using iPhone Browser: On the left pane there is a file tree.
Go to Applications -> Mobilemail.app
Rename Mobilemap.app to something like Mobilemailx.app (something different from the original name)
Click on Mobilemail.app (or whatever the new name of the folder is) and on the right pane there will be a listing of files in that folder.
Right click on Info.plist and click Save As, save it to your desktop or something. Open it in a text editor like Notepad.
Find the phrase <key>CFBundleIdentifier</key> and after it there should be something that says <string>com.apple.mobilemail</string>
Replace mobilemail with mobilemailx or whatever the new name you chose for mobilemail.app is. Save the file and drag it over the original Info.plist file in iPhone Browser. The program will replace the original file while making a backup in its default backup folder, which you can see by going to File -> View Backup Files
Re-spring your phone and it should work. You will notice that Mail is now called Mobilemailx or whatever the new name you chose was. I went to Cydia and downloaded an app called Rename to change the displayed app name in Springboard back to Mail. If you can't find this app on Cydia you will need to add http://iphone.org.hk/apt/
to your sources and then it should show up.