MDN (Message Disposition Notification):
This field requests the status of the E-mail message after delivery through the SMTP (Simple Mail Transfer Protocol) transport system. Once the message has arrived at the receiver, this data is used when the machine or user reads the received E-mail. For example, if the message is opened for reading, the receiver sends back a notification to the original sending machine or user.
The receiver must support the MDN field in order to be able to send a notification report, otherwise the request will be ignored.

To receive the TX verification mail properly, you must switch Notification in the Setup Mail TX to On.

This machine cannot receive E-mail, to use the TX Verification feature you must redirect the return notification to a separate E-mail address. You can configure Mail Address to the E-mail address that receives the notification.