In few easy steps, we will be copying Mail.app from your iPhone so that you can use it on your iPod touch as well! While it is possible
to obtain Mail.app without owning an iPhone, it may be illegal to do so because you do not have a license for the mobile Mail.app.
Jailbreak your devices
Jailbreak your iPhone (instructions coming soon) and your iPod touch (instructions here)
Install SSH and SFTP on both devices
Install SSH and SFTP on your iPhone and your iPod touch so that you can transfer files from them.
Connect via SFTP to your iPhone
Use an SFTP client such as Fugu, Fetch, or WinSCP and connect to the iPhone. Navigate to the /Applications directory.
Copy the Mail application from the iPhone
Drag MobileMail.app from the iPhone onto your Desktop.
Connect to your iPod touch via SFTP
Disconnect from your iPhone and use the same program as in step 3 to connect to your iPod touch. Navigate to the /Applications directory
Copy the Mail application to the iPod touch
Drag MobileMail.app from your Desktop to the iPod touch’s Application folder.
Restart the iPod touch
Turn the iPod touch off by holding the Lock button (upper right corner) and sliding the red slider when it appears. Turn it back on again by holding the Lock button. Mail should now be on your Springboard!