The user-friendly, lightweight, and fast email client Latest Release: 3.11.0 (News), on 20 October 2014

The Libravatar plugin

This plugin allows displaying sender profile images of the FOSS libravatar service or any compatible implementation. It also makes use of configurable disk caching to minimize network requests. Supports all relevant missing profile modes of the service, which provide alternate avatars (either dynamically generated or predefined) for addresses which don't have a libravatar (nor gravatar) profile.
Here are a few screenshots illustrating the features: