How to define inactive subscribers
Build an inactivity rule around the subscription, sending frequency and meaningful actions.
An inactivity rule should support a sending decision. Begin with what the person subscribed to and how often that subscription actually reaches them.
Define the evidence you will use
Write down the signals available: replies, clicks, preference confirmations and relevant account activity. Email opens alone are weak evidence of intent. Automated image retrieval can register activity, and a reader can consume a message without creating the signal you expected.
Product activity can add context, but it does not automatically demonstrate interest in every marketing subscription. Keep newsletter eligibility distinct from product access.
Use a window that matches the program
A monthly publication and a daily digest create very different opportunities to engage. As a planning exercise, compare inactivity after a fixed number of sends with inactivity after a fixed number of days. Inspect how many people each rule selects and what they originally expected.
No universal 30-, 60- or 90-day rule fits every program. Document why you chose your window, the signals it includes and when you will revisit it.
Separate ineligible from inactive
Unsubscribed contacts, known complaints and invalid addresses need exclusion rules. They should not enter a re-engagement campaign merely because their last interaction is old. Inactivity is a separate classification for people who still have an eligible subscription.
Make the rule reviewable
A useful written rule names the subscription, last qualifying action, lookback period, minimum opportunity to receive mail and all exclusions. Test it on a few individual records before using it to select a campaign audience.
Keep a short decision log: the old definition, the new definition, why it changed and the date. That makes later performance comparisons easier to explain.