The <head> element holds all metadata about your email, including its <subject>, <style>, and <preview>. It has the same function as the <head> tag in HTML. It must be a direct child of the <heml> tag and can only be used once.
<head>
<subject>
<style>
<preview>
<heml>
<heml> <head> <subject>Email Subject</subject> </head> </heml>