Sadly, it does not change the outcome. Even with a pure "@"@mydomain.tld, Dovecot splits it into the empty string user (%n) and @mydomain.tld as the domain part (%d).
That's a shame. Doe sit handle other quoted user names? I know it breaks on UTF-8 specific characters, but I think that is documented as unsupported.
RFC-5322 3.2.4 and 3.4.1 are pretty clear, a quoted scoring is to be treated as an atom.
“The locally
interpreted string is either a quoted-string or a dot-atom.”
That said, I'm not sure who supports a username portion in this form, so it's probably an edge-case on an edge-case.
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My main job is trying to come up with new and innovative and effective ways to reject even more mail. I'm up to about 97% now.