Groupon made a billion dollar business out of email marketing.

I don't think it is underrated by people who are good at growth. Most startups do think email is boring and rather invest in third-party platforms. This is a mistake as email is the true social network and reflects the raw habits and beliefs of users.

It is hard to do email well because there is so much noise and crap users see. I open notification emails all of the time if they are valuable to me (ergo. answer this question).

Just use the golden rule: would I want to open an email like this?