My small contribution here...
I've used Pegasus (not since about 2000, though) and TheBat! and finally settled on Thunderbird.
I still have a license for TheBat! but I didn't go beyond v7 -- I always loved its configurability and its use of FidoNet-style message quoting but I got badly burnt by a massive failure of the encryption mechanism for the Voyager variant and I decided never to trust it again.
Thunderbird's abandonment of its addons infrastructure is an annoyance -- a couple of addons for message management that I used and relied on have either gone away already or are going away as we speak (Nostalgy and QuickFolders being the most important of them) but the newly discovered ability to create virtual folders via the saved search functionality has just stopped me wondering about possible alternatives, which is why I started reading this thread in the first place!
I have tens of thousands of stored emails in Thunderbird and it copes. This is probably A Good Thing.
Consider trying out Interlink Mail & News.
It was developed after Thunderbird announced that it'll follow the webextensions route.
I've been using it for seven months now and it works perfectly. In some regards it's faster than Thunderbird (e.g. yearly view in calendar).




