Reacher wouldn't work anyways.

Because people will hate getting the reply message with the invoice, they will mark it as spam. Then after enough of this happens google and the blacklist services will treat messages from as being spam -- right to the spam folder.

As you mention,... for this to work it would require a change to the SMTP protocol to be effective. That could be done. No idea if it ever will. Reacher is not the solution though.

Exactly that. I notice also that google start blacklisting a lot of IPs/domains. I had to make some mumbo jumbo with their anti-spam filter to whitelist my IP. (DKIM, SPF, signatures from their google serves etc)

Is fucking crazy, they really push people into using the fucking gmail.