Carlo Gandolfi has developed a way to run Mail and Address Book on a memstick or USB stick, iPod or external drive.
It works as a shell script which opens the local copy of Mail using preferences stored in the external application bundle. The Portable Address Book works in the same way.
If you have a USB stick big enough, you can also store your Mail folder, com.apple.mail.plist and caches on it.
This means that take your Mail and contacts with you and get working on any Mac that has local copies of Mail and Address Book installed.
And there’s more….
However, Portable Safari, Portable Mail, Portable iCal, Portable Address Book and Portable iChat require a USD .99 donation before you can download them. Others like Portable Adium, Portable AbiWord and more are free. See the details and the scripts that makes it work on osxportableapps Sourceforge page 
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The Free Open Source Mac User Group has made even more applications portable. Their stuff is free.
Great software. I like the freedom which you get with this mobile applications. Having my favorite apps on an USB stick is wonderful.
Thank you for sharing this software with me !
Also presumably means that you could run Mail.app within a secure disk image, without the wider need for FileVault?
It is possible to sync your portable Mail with your main Mail?
Hi,
gives this the ability to launch two independent Mail.apps simultaneously?
@ Tauquil
Yes, SyncPAppX let you sync your portable apps with your main ones.
http://osxportableapps.sourceforge.net/syncpappx/index.html
@ Tekl
No you can’t, to prevent using same preferences by two Mail.app in the same time, Portable Mail check if Mail.app is already running and eventually prompt you to quit it.
Cannot download Portable Mail. Script is useless for me
Dr. Ã.Z. Györy DSc, MD, FRACP
4 Fisherman’s Pde
Daleys Point
NSW 2257
Australia