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The message could not be sent because it rejected you as the sender. Server Response: "550 SMTP Envelope Sender must be valid'. (Account: 'mail.example.com', SMTP Server: 'mail.example.com', Error Number: 0x800ccc78).
Funny thing is, for some mail servers, Outlook 2000 will process the read receipts, certain others, forget it. My troubleshooting efforts so far seem to be something with the "postmaster" account and bounce email setups being different.
Does anybody have any definitive research/answers to the issue?