XFOR: Mail Looping When First Host Cannot Be Reached

MORE INFORMATION

An SMTP host should deliver messages destined for a remote host to itself. After you perform an Exchange record query, if one of the Exchange hosts is the host doing the query, it must remove itself and all Exchange hosts with a weight equal to its own from the list.

This is done for two reasons:

1. To prevent looping.
2. To avoid message backward flow (that is, messages moving to a less preferred Exchange host).

The Internet Mail Service handles this as long as it is running on a host with one network interface card (NIC) that has one IP address.