If a message cannot be delivered on the first attempt, Microsoft SMTP Service sends it again from the Queue directory after a specified time. You can set the interval between delivery attempts. You can also designate the number of times to attempt to deliver a message. After the limit is reached, the message is returned to the sender with an NDR and copies of the message and NDR are sent to the location you designate. The NDR is placed in the Queue directory and goes through the same delivery process as messages. When the NDR reaches the maximum number of retry attempts, the NDR and message are sent to the Badmail directory.