What is in your email isn't the only data that sending one can generate. There's the IP address where it was sent from and where it's going, there's the time of day it was sent, there's a load of metadata attached they can read and glean information about you and your recipient. And there's advertisement opportunities in the interface that many services use to collect info on you. There are so many ways to collect data on you through your interactions online it's not even funny.

But yeah, keep pretending like you've found the only mail service that doesn't collect any data on you at all if it helps you cope.