Information hiding is key. I get bombarded by so many trash emails every day that I could spend weeks unsubscribing to stuff. When Gmail came out with their tabbed interface and hid all the marketing messages behind a tab, I started using email again for the first time in about 7 years. Before that it was actually less useful than postal mail because I couldn't find anything for the flood of crap.
That's something Outlook and Thunderbird don't do, is give you convenient features to hide information. Sure, you can organize things into folders or labels, colorize things for follow-up, sort emails, etc., but you're fundamentally limited by their model of "emails should be in a list, like a database. And they should be in chronological order. And you deal with them in chronological order, or you colorize them."
Outlook has added a lot of "smart" features to its inbox sorting and management. I'll just say "so far so good" but clearly there's something in the air in the space.
That's something Outlook and Thunderbird don't do, is give you convenient features to hide information. Sure, you can organize things into folders or labels, colorize things for follow-up, sort emails, etc., but you're fundamentally limited by their model of "emails should be in a list, like a database. And they should be in chronological order. And you deal with them in chronological order, or you colorize them."