The first sentence or two.
In Gmail you can read the first line of an email without even opening it. Outlook and other email programs allow you to do the same thing.
I know I make a lot of decisions as to if an email is just a pitch or has something of value without opening them.
Do you check out a WF thread by hovering over the title before opening it?
This is something you probably already know, but I've never heard anyone mention it so thought I'd share. Sometimes the simple stuff is overlooked.