Exchange server on same subnet as other servers from other domains will not transact mail with those other servers

As the title suggests, I have a block of IP addresses in which I have sat a number of servers running Exchange Server, of various versions.

They are all servers I am hosting for clients, and my Exchange Server is in the same block of IPs.

Each Exchange Server is for a different Internet domain, and works as expected, except one.

My server cannot send mail to one of the others. Investigation has found that it will not accept mail from any of the four other Exchange Servers on that block of IPs, either.

If I log on to any of the other Exchange Servers via RDP and Telnet to the rogue server (SBS 2008, Exchange 2007) on port 25 I get:

421 4.3.2 Service not available, closing transmission channel

I figured I would add all the IPs in the block to the Network tab of the default Receive Connector. After doing so I can Telent from any of the server in the IP block to the problem child on port 25 and it opens a connection.

HURRAH!

But, mail does not get through, and I get a message that it is queued for a later delivery attempt.

What else should I do?
snooflehammerAsked:
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snooflehammerAuthor Commented:
UPDATE

This is the latest bounce message (sanitised) after I have attempted to whitelist the IPs in the IP block:

Diagnostic information for administrators:
Generating server: SENDINGSERVER.sendingdomain.local
testsend@receivingserver.com
SBS2008.receivingserver.local #530 5.7.1 Client was not authenticated ##
Original message headers:
Received: from SENDINGSERVER.sendingdomain.local ([fe80::28b8:a53a:7083:343]) by
 SENDINGSERVER.sendingdomain.local ([fe80::28b8:a53a:7083:343%12]) with mapi; Tue,
 20 Oct 2015 17:01:27 +1000
From: Me The Sender <MeTheSender@sendingdomain.com>
To: "testsend@receivingserver.com" <testsend@receivingserver.com>
Date: Tue, 20 Oct 2015 17:01:09 +1000
Subject: Tester 3
Thread-Topic: Tester 3
Thread-Index: AdELBReKCB1AFGVrT4W91eiWEaUnzw==
Message-ID: <0E7D484632136044B4B37008508D34340284DF5650CA@SENDINGSERVER.sendingdomain.local>
Accept-Language: en-US, en-AU
Content-Language: en-US
X-MS-Has-Attach:
X-MS-TNEF-Correlator:
acceptlanguage: en-US, en-AU
x-tm-as-product-ver: SMEX-11.1.0.1239-8.000.1202-21890.005
x-tm-as-result: No--42.453300-5.000000-31
x-tm-as-user-approved-sender: No
x-tm-as-user-blocked-sender: No
Content-Type: multipart/alternative;
      boundary="_000_0E7D484632136044B4B37008508D34340284DF5650CACTCSERVERct_"
MIME-Version: 1.0
Andy MIT Systems ManagerCommented:
I would start by having a look at the SMTP send and receive logs on the servers. These should indicate if the receiving server has even acknowledged the connection from the sending server, which receive connector is in use, and whats actually happening during the process.

The bounceback you copied indicates that the receiving server is expecting the sending server to authenticate with it before sending email which I wouldn't expect for two un-related servers trying to send emails to each other.

Also having an SBS server on the same subnet as other domain controllers/exchange servers can cause issues as it doesn't play well with others. Ideally each server needs to be on it's own subnet/vlan and communicate with each other via the router rather than directly with each other (which if each server is for a different client) could be seen as a security risk.

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