Question : Cannot Resolve Loopback 5.3.5 Delivery Error

Alright Experts,

Here's my problem; may be an easy one for y'all...
I've successffully configured my DNS settings to send and receive, but am still having difficulty resolving the DSN (Failure) Error 5.3.5 - Loopback? All incoming email fails with this error code given.

All of the tests work fine from https://www.testexchangeconnectivity.com/Default.aspx and http://www.mxtoolbox.com with the exception of Reverse DNS lookup, which I've contacted my ISP for PTR record changes.

I have MSExchange2k7 on Win2k8... Exchange is hosted on srv02.domain.net and the DC is on srv01.domain.net. SMTP Virtual Server is also running on srv02.domain.net.

I have a single SMTP Virtual Server created with two entries under the Domain listing... one points to the Exchange server (same box), includes just a Drop Directory, and the other points to the Domain Name (domain.net) with Allow Incoming Mail to be Relayed and Forward all mail to [IP address].

The single Virtual Server has (All Unassigned) IP address: enabled... I've gone back and forth here with fixed IP to no avail.

I have disabled IPv6 in the network settings. In the Receive Connectors of the Hub Transport node under Server Configuration ((EMC), I have two entries...1) is Client (enabled) and All Available IPv6 and IPv4 assigned on port 587... 2) is the Default with the same Network properties assigned (All Available IPv6 and IPv4) on Port 25.

I'm really at my wits end here trying to chase this down... where should I be looking, and what else should I be doing? I've actually considered deleting the single Virtual SMTP Server and recreating it, but thought better of that till I received some outside advice.

So... advice anyone????

Thanks so much for your time in advance.

Answer : Cannot Resolve Loopback 5.3.5 Delivery Error

An extract from an MS article (http://support.microsoft.com/kb/284204):

Numeric Code: 5.3.5

Possible Cause: A loop-back situation where the server is configured to loop back on itself was detected.

Troubleshooting: If you have multiple SMTP virtual servers that are configured on your Exchange computer, make sure that they are serving unique incoming ports and that the outgoing SMTP port configuration is valid to avoid looping between local virtual servers. Check the configuration of the server's connectors for loops. For example, make sure that no connectors exist that have the address space of the local organization, unless you share the domain and you do not select Use DNS to route to each address space on this connector. For more information, click the following article number to view the article in the Microsoft Knowledge Base: 321721  (http://support.microsoft.com/kb/321721/ ) Sharing SMTP address spaces in Exchange 2000 Server and Exchange Server 2003 Make sure that if there are multiple virtual servers, that none are set to All Unassigned.

I would be inclined to delete and re-create your Receive Connector.