The reason the font will look different in some email clients is because not all email software supports custom fonts. So it's not a limitation of our email templates, but a limitation of the email software the email is being viewed in.
In the ones that do support it, your custom font will be shown, and in the ones that don't, we've picked "fallback" fonts that match your custom font as closely as possible. Of course, if the fallback font doesn't match the custom one at all, please let us know so we can see if we can find a better fallback for it.
If you want everyone to see the same font, no matter what, then you can use one of the following:
Arial or Helvetica
Times New Roman
Personally, I would use the custom font that matches your site the closest and then allow the fallback fonts for people that don't have a supported device, as they would never know what the custom font version would have looked like anyway ;)