Context-awareness is a key technology for adaptive software systems. While past and recent research effort investigated the gathering of low-level information, implementation- and domain-driven approaches for context frameworks and conceptual models of context, vertical approaches combining the aspects of modeling, and implementation are still missing. In this paper, we propose a scalable and distributed context service for integrating heterogeneous context sources in large-scale mobile environments based on a layered context model enabling the modeling and support of multiple domain-specific context models. We implemented an adaptive multimedia messaging system on top of a 4G mobile communication testbed which supports the context-dependent selection of codecs and communication modes to validate our approach.