I noticed today that some text Gmail (“It’s been a long time since …”) shows in list view doesn’t appear in the email itself:
I was wondering if there was an SMTP header I didn’t know about. As it turns out, there’s not. Instead there’s a hidden paragraph in the email body:
<p style=3D"max-height: 0; font-size: 0; l=
ine-height: 0; margin: 0; overflow: hidden;">It's been a long time since we=
've seen each other!</p>
This means the paragraph doesn’t show when you read the email. A bit of searching later and I discovered it’s called the “Email Preheader”.