[vidarh]

That's an argument for writing an email client, not for writing a proprietary service that requires them to learn an entirely new system and convince all their contacts to move.

Put another way: Nobody but me needs to understand or change behavior to make [e-mail] work, and I don't have to configure anything - I can just make them up whenever I want (everything after the +) and set up a rule of/when I decide I want them in another folder, or want to do other things with them.

There's a case for better UI for that. There's not a case for a messaging system that isn't email, that requires my contacts to change behaviour, to do that.