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.
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Mail no longer opens when I view contact info or look up addresses in Maps, which means contact and address info loads much quicker and more RAM is available. However it also no longer allows me to email people directly from my contacts list and it probably won't hook to email addresses for locations in Maps. Personally this doesn't bug me because I can just copy-paste the email address into Mail by myself, but it's worth keeping in mind.
This is really just a simple workaround until someone who is much more savvy than me figures out how to block the underlying hook that is opening mail in the first place. By renaming the app to something else the watchdog that opens Mobilemail.app (some people just refer to it as mail.app) will not be able to find the newly-named mail program and therefore will be unable to open Mail.
Hope that helps. And thanks to pablogiorgini for posting the hack