I get a Mail Delivery Status Notification (Delay) whenever I accept the read receipt which the sender includes in his mail. After I click on the "yes" and read through the mail. The following mail gets delivered to my inbox.
This message was created automatically by mail delivery system.
THIS IS A WARNING MESSAGE ONLY.
YOU DO NOT NEED TO RESEND YOUR MESSAGE.
The original message was received at Wed, 08 Jul 2015 23:32:57 -0700
----- The following addresses had fatal errors -----
[Status: Error, Address: <richter@localhost>, ResponseCode 421, , Host not reachable.]
Message will be retried for 4 more day(s)