There is a variety of desktop mail clients, but the vast majority (of cross platform _and_ open source) is small-ish unkown-ish programs. The plugin system is i

There is a variety of desktop mail clients, but the vast majority (of cross platform _and_ open source) is small-ish unkown-ish programs. The plugin system is in my opionion important (like for browsers) functionality, so popularity of a client is a significant factor, in order to have plugins available. Evolution is possibly the only valid competitor, however, it's not cross-platform as Thunderbird.