If the source to SMTP server is reviewed that it does send the email (backed up with test results) and as part of the sending process it adds a hash value of email to the blockchain. I think that allows peer verification, please explain where I am wrong. I am certain I have made mistakes somewhere and I am still coming up to speed with this whole decentralized blockchain approach.
You might think that you could try to guarantee than an email has actually been sent if the "receiver" indicated that they had received it through another AM, however, just because they didn't receive it doesn't mean that it was sent - again - "there is simply NO WAY to do this - so please stop trying now" (you are wasting time just as much as you would trying to solve "the halting problem").
Besides, there are many other problems. Most of these emails would end up in spam folders anyway