Even if it would be hard to modify the SMTP and POP protocols it can be still done with in the same server/domain, e.g. gmail.com. Unless you can make it unive

Even if it would be hard to modify the SMTP and POP protocols it can be still done with in the same server/domain, e.g. gmail.com. Unless you can make it universal, i.e. across all domains, it's unlikely to catch on. Microsoft had (still has?) a similar feature in Outlook, but it was dependent on the recipient also using Outlook on the same network (or something like that) and few people used it. FWIW, I'll never forget the advice I got about email from a mentor at an i-bank where I used to work: don't write anything in an email that you'd be embarrassed to see show up in the Wall Street Journal.