A lot of the things people suggest as replacements for SMTP could be done with SMTP if email signing became standard. You could have closed message groups where

A lot of the things people suggest as replacements for SMTP could be done with SMTP if email signing became standard. You could have closed message groups where you only have emails from people you have corresponded with before. So you could have a family INBOX, a company INBOX and a friends INBOX. Getting added to a particular INBOX would involve some communication out of band. This could be done in a very intuitive way. People would have no problem understanding the concept of a private INBOX. It would totally allow man in the middle attacks but spammers/scammers generally can not do such attacks and would not have any financial incentive anyway.