The note made earlier in this topic that the San Francisco fonts are specifically designed for the screen is wrong. The entire SF set are standard Postscript OpenType fonts. The same as any other such font. The reason you can't use them is what has also already been mentioned. They are intentionally blocked. All of the fonts are labeled as System Font, such as in this UltraLight version.

The OS is simply hiding anything with System Font as part of the internal names. That they are seen anywhere at all (such as in Mail), would be a mistake made by the person writing the code for that app.
The set exists as display parity to iOS, which has been using San Francisco for a while. I don't recall which version of iOS it debuted in, but I think iOS 7.
Tom Gewecke made an excellent point. If you were allowed to use it in an email, anyone running Yosemite or earlier, or using Windows, would not see that font. It would have to be substituted.
I also can't agree more with other statements about using it at all. It is only sightly different from Helvetica, Arial and other sans serif fonts you already can access. I don't get why folks think this font is so much better.