What fonts can be used?
The short answer is that, generally speaking, only standard web fonts should be used in emails. The way that fonts are handled in email is that you can't pull in external web font files (google fonts, etc.) the way that you can on an website - the only fonts that can be used are fonts that the user already happens to have on their computer.
Now, having said that, there are more interesting fonts commonly available on the average user's computer these days, so you can always try to use a more interesting font and just fall back to Arial or Sans-serif.