People receive a tremendous number of messages through mobile instant messaging (MIM), which generates crowded notifications. This study highlights our attempt to create a new notification rule to reduce this crowdedness, which can be recognized by both senders and recipients. We developed an MIM app that provides only one notification per conversation session, which is a group of consecutive messages distinguished based on a ten-minute silence period. Through the two-week field study, 20,957 message logs and interview data from 17 participants revealed that MIM notifications affect not only the recipients' experiences before opening the app but also the entire conversation experience, including that of the senders. The new notification rule created new social norms for the participants' use of MIM. We report themes about the changes in the MIM experience, which will expand the role of notifications for future MIM apps.