The client has the option to use the email template layout for future reference so it's important to implement editable code in the emails so it allows them to change the content without messing the template up.
Mailchimp provides already made templates for Desktop and mobile email clients. If you are creating your own template. Check out Mailchimp Template Language of how it works.
Check out the Oracle documentation of how it works.
To add the commands, you can read this support article.