The health of an Open Source ecosystem is an important decision factor when considering the adoption of Open Source software or when monitoring a seeded Open Source project. In this paper we introduce responsiveness as a qualitative measure of the quality of replies within mailing lists, which can be used for assessing ecosystem health. We consider one specific metric of responsiveness in this paper, and that is the response time of follow-up messages in mailing lists. We also describe a way for characterising the nature of communication in messages with short and long response times. The approach is tested in the context of the Nagios project, and we particularly focus on the responsiveness for contributors acting in their professional roles as core developers. Our contribution is a step towards a deeper understanding of voluntary support provided in mailing lists of OSS projects.