Yes! To do this, you add the HTML version of your email to the "HTML Body" field on your Zap template.
You need to do this in addition plain text version of your body. The plain text version is then a fall back if the receiving client can't accept HTML.
For tips on writing HTML, check out W3 Schools here: https://www.w3schools.com/html/html_basic.asp
If you get an error with "503 5.5.1 RCPT" in the message, this likely means that the email address the zap was trying to send to was empty or invalid. Double check that you have a valid email address in your sample data and task history for that zap, and make changes to the configuration of your zap if you don't.
When using SMTP by Zapier, the emails are actually being sent by your mailserver. Zapier communicates with it via SMTP to deliver the data from our system, but your mailserver is ultimately sending these emails. You should review your SMTP logs should you need more information about delivery status.