I've definitely experienced this on the receiving end, both with startups and new products, but also with random mailing lists that only send out updates sporadically.

A good rule of thumb for ANY mass email communication is to lead with a short explanation of why they're receiving the email: "Thanks for signing up for updates from Newproduct, a new platform for knitters to connect online."

Remind them 1) they signed up, 2) what they signed up for (updates), and 3) who you are (descriptively).