Assuming you're asking what to include in a launch email for a new product or service:
Subject line: Introducing [name of product or service]
Body:
Hey there!
We're excited to introduce our newest product or service, [name of product or service]. [Brief description of what it is and what it does.]
We created this product or service with you in mind, and we hope you love it as much as we do. [Include a personal story or specifics on how this product or service will help the reader.]
Here's what you can expect