Once again, thanks for clearing things up.
So I guess I'm confused. Isn't the only way to have an empty sender having no "MAIL FROM" during the smtp session?
No. Here's another way:
MAIL FROM: <>
Does courier accept smtp sessions where there's a RCPT TO but no valid MAIL FROM?
Courier requires a MAIL FROM. A "MAIL FROM: <>" specifies a valid, empty sender.
"authenticated" delivery system. However, I do need to let through messages from postmaster/mailer-daemon and any other valid system-generated messages, including DSN and the like. But if I also allow any message with an empty sender, then it might defeat the purpose of the system.
Valid DSNs use an empty envelope sender, as defined above. There is a small minority of broken mail servers that use some randomly-generated, invented return address for delivery status notifications. They can be safely ignored, as hopelessly broken.
I thought that the $SENDER variable was set to whatever the "MAIL FROM" was set to during the smtp session.
Correct. If "MAIL FROM" specifies an empty return address, SENDER will reflect that.
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