There was a company that offered a product for Windows 95/98 users around 2000 that did that, except it worked with existing email clients instead of requiring new ones. It worked reasonably well, as long as the user understood the limitations such as the metadata issue.
There was no common extension interface for the major email clients (and many didn't have any extension support), so their biggest technical challenge was finding a reasonable way to hook in between the user's email client and the network.
Their first approach was to have their program run as a daemon that implemented SMTP and POP3, and the email client's proxy settings were changed to use the daemon as a proxy. Their setup program recognized the top email clients and know how to change their proxy settings. For other clients the user might have to manually change their proxy settings.
Later they switched to using the LSP interface to hook into the network stack to intercept and modify email regardless of what email client was being used.
There was no common extension interface for the major email clients (and many didn't have any extension support), so their biggest technical challenge was finding a reasonable way to hook in between the user's email client and the network.
Their first approach was to have their program run as a daemon that implemented SMTP and POP3, and the email client's proxy settings were changed to use the daemon as a proxy. Their setup program recognized the top email clients and know how to change their proxy settings. For other clients the user might have to manually change their proxy settings.
Later they switched to using the LSP interface to hook into the network stack to intercept and modify email regardless of what email client was being used.