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").
Understand that this is why you don't want to mix up stuff from "within the blockchain" to stuff that is "outside the blockchain" as only the former can ever "be proven by the blockchain".
Maybe proving that reviewed source code was invoked is useless in the case of email. Its advantages are that it is easy to verify, easy to understand what it does. Its purpose is NOT to send emails, but to work out architecture issues for plugins
A txid in the blockchain only means that the plugin code was executed. That is probably not enough, without error handling, etc., but this is first proof of concept version that is meant to be evolved with more and more robust solutions. Of course, we wont use it for actual sending of email
What I am looking for in the blockchain is that code was executed without error. For example, if the code that was executed is code that verified the existence of a transaction in the bitcoin blockchain, this mechanism can be used to step through the cross chain algos.