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> You never really could. Participating in public email exchange requires that the sender can resolve then "fully qualified" domain in your return address.

I thought you could send from <> for things that shouldn't bounce.

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If the return path is null that just clarifies unattended notifications should not be returned. The mail is still considered to be sent by the transmitting system, and when no mailbox is specified, the implied envelope sender simply defers to the "postmaster@your-ehlo-fqdn.example" address. (Accepting mail at the "postmaster" mailbox is a mandatory part of SMTP, as is mentioning your fully qualified domain name in the "Hello" message when initiating the session. Public mail exchanges are free to, and often do, reject clients that submit anything other than resolvable domains there. Same with clients that use <> for applications other than those very limited "notification about specific quoted/referenced message" scenarios where the standards mandate <>.)

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