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?

I don't think it's underrated, anyone I know working on growth has a strong email component as part of the underlying strategy.

Regardless if it's product features, or other types of growth tactics, email is usually part of it.

As much a people say they hate email, they still open and if you're good, respond to them.

Almost all growth tactics at Clarity have included an intelligent email component.

It does seem to have taken a little bit of a back seat in the eyes of most startup founders, I guess it's not sexy like Facebook boosted posts or Tapjoy incentivized installs... but it's one that still works exceptionally well.

Moreover, of the startups I've invested in this year, 2 of the 5 that are making profits do it through an email list they curated from app installs and then sell access to that list to other companies who want to reach the customers they have.

When active users have opted-in to receive e-mail communication, e-mail marketing is an extremely effective channel for customer retention.

It isn't as effective for user growth via purchased e-mail lists because recipients are not users yet and may not have opted-in - they haven't developed a relationship with your business, so when they receive e-mails, they aren't recognizing them as coming from a trusted source.

Every business should be utilizing e-mail marketing and marketing automation to communicate with their users / customers and improve retention.

Email marketing is a great way to engage your customers, keep them current and also make them loyal.

Email is a great way to start and nurture relationships with your customers, but don’t just send updates on your products. Give them a look inside your company. Share customer interviews and case studies. Write blog posts, and record short videos that will help your customers.

Maybe you have new content to share or you want to check in with current customers because you haven't heard from them in a while. Keep your emails personable and quick to read.

(Here are some more tips on building customer loyalty with email marketing and beyond, if you want other ideas: http://www.fieldboom.com/blog/customer-loyalty/.)