We have a Win 2003 Server running ISA 2004; acting as a gateway beetween our Exchange server and the outside world. On this gateway, IIS is set to forward all outbound email to a smarthost (our Anti-Spam provider) which is a DNS name. It was all working fine and no changes have been made, but one day outgoing email stopped, the email is queuing and the System Event Log full of ID 4000 messages like this:
"Message delivery to the remote domain 'hotmail.com' failed for the following reason: Unable to bind to the destination server in DNS. "
Inbound email is still fine (because a Remote Domain smarthost points internally by IP), and from the server you can still NSLookup quite happily, and Telnet on TCP 25 to the smarthost. So why has this stopped working? and why is IIS looking up the DNS for every outbound email when it's all pumped to a smart host anyway? Changing the smarthost to an IP address gets round the problem as there's no DNS lookups, but we need to use the DNS hostname because it points to a cluster of servers.