In:
/app/design/frontend/{vendor}/{theme}/web/css/source/_variables.less
Change:
@font-family-name__base: 'Lucida Grande';
@font-family__base: @font-family-name__base;
If you don't have this file in your theme, copy it from the parent theme (Magento_Blank).
This will most probably change the font in your entire Magento application and emails.
If you don't want to change the font for your shop or if the font isn't changed in the emails after your change, check this file:
/app/design/frontend/{vendor}/{theme}/web/css/source/_email-base.less
(again copy it from the parent theme if you don't have it in your theme)
Search for "font-family" and change all instances to 'Lucida Grande' or if you use the same font throughout your shop, change it to @font-family__base
.
Also make sure the email template you are sending has the header
template included. You can add it with {{template config_path="design/email/header_template"}}
at the top of your email.
EDIT
You should stick to Magento's default way of implementing font & styles to emails, because different email clients have different ways of "reading" the styles. For example Gmail reads only the inline styles, while Outlook reads the non-inline styles.
So if you only use one way, many email clients won't receive the emails correctly.
Not to go into details, because email styling is a broad topic, Magento has the basics covered so if you're using Magento Blank
as your parent theme, or the same "email style import logic" as Magento Blank, this is the way to go.
EDIT 2
If you are using Luma theme, and you are extending it. Just copy /vendor/magento/theme-frontend-blank/web/css/source/_email-base.less
to /app/design/{vendor}/{theme}/web/css/source/_email-base.less
.
Luma isn't intended for extending, because it depends on Magento_Blank being the parent. And by extending it, you effectively removed the Blank theme as the parent and all of the stuff Luma depends on.