I think the real missed opportunity here was not bothering previous customers with marketing emails at all. If I want to use your service, I'll subscribe to it. As someone who takes the structure of their inbox seriously, receiving promotional mail of this kind would result in a very quick unsubscription from all their emails.

Lest anyone think this is "look at all the crappy emails Calm sent me that I didn't want what schmucks", it's really some marketroid explaining all the ways Clam should have been emailing them to maximize crap marketroids think are important. Notably, they think a user unsubscribing from a service is certainly no time to stop marketing that user...it's only the beginning of marketing opportunities.