The "email-renew-interval" setting:

If this setting as an integer N that is 14 or greater then email notification is suspended for subscriptions that have a "last contact time" of more than N days ago. The "last contact time" is recorded in the SUBSCRIBER.LASTCONTACT entry of the database. Logging in, sending a forum post, editing a wiki page, changing subscription settings at /alerts, or visiting /renew all update the last contact time. If this setting is not an integer value or is less than 14 or undefined, then subscriptions never expire.