I have used the lato family lato font in frontend. I need to add this font-family in email template also.
Set the font family for the relevant elements to be Lato and then Include the Lato font in the head of your email. You can do this with Google fonts Google font - Lato
If you don't do this then only users with Lato installed will render the email correctly
I was trying to post in comment section but couldn't due to reputation.
I think in Transactional email->Edit email template and you can edit the custom css and then call that template in sails email if you already included google reference in your head section.
body style="background:#F6F6F6; font-family:'lato', sans-serif;"