Custom fonts can be added to HTML emails either by hosting the font directly on your server (ensuring the fonts is shareable and in the correct formats) or using a third-party font service such as Adobe or Google Fonts.
If you do use custom fonts in your HTML email be aware that some email clients do not support custom fonts, and in these instances, you will need a fallback font.
HTML email fallback fonts need to be system default fonts, so one of the below:
- Palatino Linotype
- Times New Roma
- Comic Sans MS
- Lucinda Sans / Console
- Trebuchet MS
You can see the full list of fallback fonts here: https://www.w3schools.com/cssref/css_websafe_fonts.asp