I wanted to inform you that adding custom fonts to Acymailing is not possible. Although, you can use web fonts by including custom CSS; it is not recommended as most email clients do not support it. The best approach would be to use the web-safe fonts available on Acymailing.
Additionally, I came across an informative article by Litmus that I believe you may find useful: "The Ultimate Guide to Web Fonts." Here is the link: https://www.litmus.com/blog/the-ultimate-guide-to-web-fonts/
I found a helpful resource for checking which email clients support custom fonts. Here is the link: https://www.caniemail.com/search/?s=google+font