Allow Custom fonts - for improved readability and accessibility (and power users)
Provide the option of custom fonts on Mobile/Web and font size control.
This technically can fall under parts of the "theme", however it is also part of accessibility.
As a User, I want to have the most clear fonts possible to make reading my emails easier, so that I use this app more.
As a power user, I have seen people struggling to find extremely readable fonts like the old Blackberry Font from the BB-Alpha font.
Whilst you would attract more business/power users from this throwback font, I think it might be easier to let them choose/add the font for themselves and simply offer the option of some defaults, but let them import/embed a font in their application.
I'm saying this because I would pay an extra fee to allow custom fonts to make my experience even better on Android tablet and iPhone.