Thunderbird's kind of crappy out of the box, but the best part of it's Moz DNA is being highly customizable and extensible.I've looked at just about every optio

Thunderbird's kind of crappy out of the box, but the best part of it's Moz DNA is being highly customizable and extensible.I've looked at just about every option for mail clients on linux and haven't found one with what I would consider a 'modern' feature-set, largely because the open-source model mostly precludes the sort of middle-ware servers which commercial mail clients on other platforms use to provide "magic" features. The next best thing, then, is highly-configurable mail flow control, which, with the right extensions, Thunderbird can get mostly right.