Follow one of the examples:
YourCompanyName: We develop power systems for IoT
YourCompanyName: Built by ex-CTO of SpaceX
YourCompanyName: At $50K MRR & growing 250% per month
YourCompanyName: Introduction from Person X
YourCompanyName: Ultra-low-power IC investment opportunity.
Do not overuse business jargon or rarely used acronyms and abbreviations.
Use metrics to show that you are actually hitting impressive numbers.
Investors usually care about such metrics as Monthly Recurring Revenue, Compounded Monthly Growth Rate, Customer Retention Rate, etc.
Be warned: if your growth is not impressive or you focus on the wrong metrics, your email might end up in the trash.