Earlier quoted context omitted.
'@' was used to mean 'at' or 'each' for a long time before email addresses. Often in pricing. '12 @ $3' means twelve units priced at $3 each. It's referred to sometimes as 'commercial at', including its Unicode name. https://en.wikipedia.org/wiki/At_sign It was used as 'at' in computing in Algol 68 where it was a shorthand for a keyword 'at'. It has a different application as 'at' in Dyalog APL.
It wasn't remotely as familiar to a broad audience back then as it is now, though. Considering that people accept its non-idiomatic use for website user names even now (which is the same as what I'm proposing no less), I don't think this would be a problem.
https://com.google. for the main page
https://com.google.mail for gmail
https://com.google.mail/localpart for one's own gmail account
mail://com.gmail/localpart to send mail to what is currently localpart@gmail.com
Alternately, we could continue to use '@' and put the localpart first. The URL method of specifying a username or username and password does this for HTTP/HTTPS.
http://username:password@com.site/login
And currently we use mailto:localpart@example.com but we could use mailsend:localpart@com.example instead.