One of best email marketing practices says that you need to obtain the recipient's permission before sending emails to them. On the other hand, if you send relevant emails to your customers who purchased your product or used your service in the past, you may not worry about whether they want to receive your emails or not. It's supposed that nobody will complain about a relevant email even if they did not opt-in for it.
It sounds promising and opens more potential to email marketers eliminating the risk to be flagged as spammer. Let's dig deeper and see if you can really ignore the permission element in your email marketing campaign.