Return receipt

When you send a mail 'requesting' return receipt, it puts a header for
Disposition-Notification-To and if you will examine the mail you
received which you sent yourself with the receipt request, you should
find that header.

How Tb handles its behavior toward you the recipient when it receives an
email with that header depends on the settings in the mail account in
the return receipts section.  The default setting is to use the global
preferences which can be configured to never send return receipts or to
ask or to always send return receipts for particular kinds of addressing.

Disposition-Notification-To: "x" <x@x>

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Delivery Status Notification (DSN) message

delivery status notification

Thunderbird adds a header to the message. Older mail servers would send
delivery status notifications back when the message was received, modern
mail servers generally do nothing of the sort unless explicitly
configured to do so.

You get a notice that the message has been received by a server.
However, the DSN could be from any of the servers along the path to the
destination. So you do not really know if the message was successfully
delivered to the recipient.

notifications are sent: success, failure, delay, or none.

http://www.postfix.org/DSN_README.html

sendmail -N success,delay,failure ... (one or more of these)

Server show

DSN capabilities

- Sender extensions
- Recipient extensions

RET

FULL
The former means that the complete message should be included in the error message
HDRS.
instructs the server to only return the headers of the failed mail.