tl;dr
Retention is not only email marketing - you need to think omnichannel and holistic
Steps that need to be combined and invested in: Acquisition -> Activate -> Retain -> Resurrect
Branding is most crucial, especially commodity products where you can't differentiate from your competition.
Holistic Retention Approach the best brands follow:
Acquisition: acquiring the right people that have the highest probability of turning into loyal customers with massive CLVs
Activate: ensuring your customers have the best possible experience with your product to get them to buy again
Retain: nurturing repeat customers into loyal customers from churning
Resurrect: getting churned customers to come back and by giving them new reasons to buy from you again
What's having a massive impact on all this: Branding! It's crucial for retention, especially commodity products where you can't differentiate from your competition.
More on the financial impact in this article/video: The Economics of Retention.