A collection of thirty national flag icons for Mail.app on DeviantART 
gives you the chance to get all patriotic with your email.
The USA, UK, Australia, Canada, China, Colombia, Denmark, Egypt, France, Germany, Greece, India, Indonesia, Ireland, Israel, Italy, Jamaica, Japan, Lebanon, Malaysia, Mexico, the Philippines, Poland, Russia, Singapore, Spain, Sweden, Switzerland, Thailand and the United Arab Emirates all feature.
Flags are not a natural fit for Mail’s stamp icon, but the artist has turned the images out nicely:
The images are PNG files, but can easily be converted into ICNS format with the utility img2icns .
And changing the icon itself is easy.
I’ve added the collection to the list of alternative stamp icons for Mail.app, making a total now of 284.
Is there an easy way to back up the original logo should I change it, but then want to revert?
I guess it depends on what you mean by easy.
The original icon is stored in the resources folders of Mail.app’s package and is called “app.icns”.
Is that helpful and easy for you to find?
Yes, that should be enough to get me through. Thanks for the pointer!