If you’re sending a text-only campaign where reporting on open rates isn’t possible, you’ll have to rely on data like tracked links, conversions, and web site activity. Also. sign up for feedback loops from the e-mail providers, so that every time one of your subscribers marks your messages as spam, you can receive a copy of that message with the subscriber’s e-mail address.
Use this data to help determine the right time to send a win-back and re-activation campaign to your subscribers.
For example, a recent analysis I did for a major retailer showed that complaints were low until subscribers approached their one-year subscription anniversary. This indicated a perfect time for them to send a win-back campaign to their subscribers, which they did with very positive results. Even better, it’s not dependent on what format you send your e-mails in.