envelope-to is used when sending an email to more then one person and you only want that person to see thheir own email address ...
The To: header includes everything in the To: line of the email, as it was originally written by the sender. The Envelope-To: header includes only the single address that a copy of the message is destined for. If mail is sent with a To: line of "
[email protected],
[email protected]", Then joe gets a copy with an Envelope-to: header of "
[email protected]", while jane gets a copy with an Envelope-to: header of "
[email protected]".