You tested your email and the font family is not the same as what you designed in the email editor. Or, you do not see a font family you commonly use available in the email editor.
The font selections in the drag-and-drop editor are the universally accepted fonts for email clients. We limit font selection to those that are universal in an effort to ensure the display of the end recipient is what you intended.
If the font changed in your test email, it's likely that your email client does not recognize it, or does not have rendering rules for it.
Use of fonts not available in the drag-and-drop editor (or hard coding custom fonts) may result in recipients seeing font as their e-mail client's default, or worse - being unable to read the text at all because their system could not render the font family.