Betterbird wouldn't exist without thunderbird though. Same with all those firefox forks people are recommending, if Firefox goes under most of those would go under too.
Thunderbird really is terrible, but for some reason it never occurred to me to find an alternative. Just commenting here so I remember to look into Betterbird.
The primary reason I hated Thunderbird, was because it would always drop authentication whenever I tried sending e-mail from my Outlook address. Yeah I can see mail coming in for my Outlook e-mail, but I also wanted to send too and I always had to go over to Outlook and deal with the awful and confusing UI to send mail.
I had just tested BetterBird's capability by sending an e-mail from my Outlook address to one of my GMails and no issues. So, fuck Thunderbird.