I found where it is stored, created some of my own, but it isn't intuitive and certainly not fool proof The basic templates are stored here: /Library/Application Support/Apple/Mail/Stationery/Apple/Contents/Resources/ Each sub directory has a TableOfContents.plist and each stationery has a UID. The first thing I did was copy the /Library/Application Support/Apple/Mail/Stationery/Apple directory to /Library/Application Support/Apple/Mail/Stationery/Mine. This resulted in two of everything. I have been experimenting with one of the Photos templates and had some success. The file content.html has the basic html, however I have had mixed success replacing the default photos. Apple has created some magic that I don't understand. I replace the three files placeholderX.jpg with three of my own, the result was good for two, but it went off and found a completely different picture for the third. This is clearly a great opportunity for an application to create stationery.