[SyneRyder]

I took a look, and it does seem many of Postbox's features are renditions of Thunderbird addons, just with a slicker UI. Nostalgy has clearly been a big influence, and possibly Expression (though both are inspired by Gmail). I haven't found an addon for the focus sidebar, but I bet that's out there too.

My only issue is that during the years I used Thunderbird (pre Postbox) & exploring the addons, I never even knew about Nostalgy or Expression. For me, it's worth $20 for someone to design a pre-loaded Thunderbird with the best addons & make it part of the default UI & support docs. I would have easily taken $20+ of billable time just to discover & decide between the TB QuickMove and Quick Folder Move addons.

[But I'm really glad Thunderbird can do this, now I can recommend Nostalgy & TB to folks who don't want to pay for an email program.]