One of the most important startup lessons to learn, in perpetuity, is not to underestimate email.

I learned this the hard way when I was running a startup 15 years ago (we used sign in with Twitter and didn't required people give us their email address) and it's still true today.

Email is almost universal, mostly free from gatekeepers and is an incredibly effective way to keep your product relevant in a way that doesn't depend on your users remembering to visit your site or open your app.

There are people out there who don't use email, but they tend to not be people who spend serious money on the kind of products most startups are building.