When would these alerts be sent? I'm thinking we want address validation on the creation of the rules (*send rules and connected data). The from address is the person updating the rules, right? One step is normal validations that would give errors on form submittal. You could also have an info alert (as opposed to an actual validation error) that says what the Wagn user cardname is for the email address (if any). Sending an email to the one making the update doesn't give any new information, they have to manually validate the email is correct. Sending to the recipient may be confusing because they don't know why they are getting it. Could do it with appropriate wording and options to redirect it or suppress it.
Every time a flexmail event is triggered. The message would either be that the mail was sent, or that it wasn't, ideally with something helpful about why. Thinking now it probably doesn't make sense to send it to the from address (may not be valid, and even if it is, may be someone who doesn't want to get emails) Emailing _user+*email should be enough, and you're right it doesn't add info but it's a good backup in case they close the tab before getting the confirmation/failure alert. Definitely agree re not sending to recipient :-).
For an initial implementation at least, I'd hope to avoid adding configuration options (eg to redirect/suppress this message as you suggest).