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 and 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 and the DC is on SMTP Virtual Server is also running on

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 ( 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.
Who is Participating?
Alan HardistyConnect With a Mentor Co-OwnerCommented:
An extract from an MS article (
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  ( ) 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.
Alan HardistyConnect With a Mentor Co-OwnerCommented:
Once you have deleted the receive connectors, please setup a new one using the Exchange Management Shell:
new-ReceiveConnector -Name 'Default Receive Connector' -Usage 'Internet' -Bindings '' -Fqdn ''
Then test mailflow please.
LowboAuthor Commented:
No joy there... same 5.3.5 error. Internal email is fine of course; external is no-go. As I noted previously, only a single Virtual Server is use...

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Alan HardistyCo-OwnerCommented:
Did you restart the Microsoft Exchange Transport service before testing?  If not, please do so and if that fails, please restart the server (just in case).
LowboAuthor Commented:
Thanks Alan... yes, I had restarted the transport service but did not re-boot the box. I've done that as well; still no joy.

It's getting late on this side of the world... I'll check in the AM for possible additions to this question.
Alan HardistyCo-OwnerCommented:
No problems - will see if I can come up with a few things to check and post for you.
LowboAuthor Commented:
Thanks Alan.... the issue is resolved. I beleive at this point the loopback error was associated with the SMTP virtual server in the IIS 6.0 console.

I actually had to go backwards before moving forward. By going back to a previous restore point I had created, and finalizing the deployment of Exchange, the issue cleared itself... during the 2nd deployment I 'did not' configure the SMTP service on the Exchange server and allowed the default Send/Receive connectors to do their thing.

I suppose you should get at least partial credit for trying to support me???
Alan HardistyCo-OwnerCommented:
Excellent news - well done.

In terms of points, that is up to you. If I helped you resolve it then great but if not then no need.
LowboAuthor Commented:
Although associated with the SMTP loopback, the support feedback only pointed me in a direction.
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