PostBox is an incredible proprietary fork of of the Thunderbird project. It provides a whole lot of functionality missing from Thunderbird and actually gets threaded conversations to work properly.

 

Only downside is that it isn't 100% compatible with Thunderbird extensions, which apparently require a small modification to work (according to the web site)

 

So, assuming this page is truthful and it's not that big of a deal, I'd request a compatible extension for PostBox.

 

Thanks.