Get to know them before trying to sell them anything.
Since you have a large user base, build cohorts of people (let's say groups of 5k people), and send them a fake personal email related to the brand to:
- do they remember the brand?
- why did they sign up in the first place?
Doing this, you'll be able to better understand their interest.

The idea to split the user base is for you to test several approaches. You'll have to use some analytics tool to track these people (email open, engagement etc...).

We can discuss this further if needed.