Hi, I saw a post from 2018 where this same issue was happening, but the support person helped her without providing a general "fix" for me to try. Basically, the font changes entirely when I send a test email to myself. And I want to make sure it won't do that when I send it to a couple of thousand people.
Here are some photos to show what I'm seeing on my end. The top image is the email in editing mode and the bottom is once it's delivered in my inbox.
Thanks for reaching out to the Community! When your readers view your email on their computer or mobile device, their web browser and email client are reading code that tells them which font to display. If the font used in your email isn't available, their browser or email client, and sometimes even the computer or mobile device, will substitute it for a font that is available, which can make your email design look different than you intended it to.
Email-safe fonts have the highest likelihood of being installed on the broadest array of computers and devices, and being recognized by browsers and email clients. Using them in your emails means that your email font is less likely to be altered. The following fonts are considered to be both email and web-safe:
The Thasadith font style you chose in the edit mode will display for some people, but if you want to make sure everyone sees the same font I recommend using Tahoma or another selection from the list above.