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I am currently experiencing an issue sending email to one specific domain. We are running Exchange 2007 (recently migrated from 2003). We are able to successfully send email both internally and externally to any other domain. The emails are queued in a DnsConnectorDelivery type queue with no Last Error message.The server is able to resolve the MX record correctly. The SMTPSend Log reveals the following:

2009-10-21T06:37:47.265Z,SMTP to Internet,08CC201AD304F254,0,,x.x.x.x:25,*,,attempting to connect
2009-10-21T06:37:47.281Z,SMTP to Internet,08CC201AD304F254,1,,x.x.x.x:25,+,,
2009-10-21T06:47:48.038Z,SMTP to Internet,08CC201AD304F254,2,,x.x.x.x:25,-,,Remote
2009-10-21T06:47:48.038Z,SMTP to Internet,08CC201AD304F254,0,,y.y.y.y:25,*,,attempting to connect
2009-10-21T06:47:48.054Z,SMTP to Internet,08CC201AD304F254,1,,y.y.y.y:25,+,,
2009-10-21T06:57:48.436Z,SMTP to Internet,08CC201AD304F254,2,,y.y.y.y:25,-,,Remote

...with x.x.x.x and y.y.y.y being the correct first and second MX records for the domain.

From my desktop (but a different external IP address), I am able to Telnet into x.x.x.x:25 and y.y.y.y:25 and send an email. When attempting to Telnet from the exchange server, it just waits after issuing the 'OPEN x.x.x.x 25' command. I am able to Telnet to other SMTP servers from our Exchange server successfully.

I'm working with and admin on the other domain to determine if our IP is somehow blacklisted, but is there anything else that would cause these symptoms?

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up vote 1 down vote accepted

As it turns out, there is a hotfix for this issue:

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is it working now? – Saif Khan Oct 22 '09 at 17:43
This hotfix resolved the issue, yes. Sorry for the ambiguity. It can be applied by running: netsh interface tcp set global autotuninglevel=disabled I haven't researched what autotuninglevel is, but it appears to have been the culprit. – Joe Oct 22 '09 at 21:37

If you can't telnet from the exchange box to that specific IP/SMTP then the problem could be on their end or on the path to their end. I had this issue twice

  1. The problem was with the ISP on the other end.
  2. I was using OpenDNS and it was blocking that specific IP. I added an "Always Allow" and that worked.
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I was able to trace all the way to their IP, so my belief is that ti is not ISP related. – Joe Oct 21 '09 at 19:32
where is the exchange box getting it's dns from? does a simple nslookup give the correct IP on the exchange box? – Saif Khan Oct 21 '09 at 21:10
Yes it does. I know from the logs that it is, in fact, resolving to the correct IP addresses. – Joe Oct 22 '09 at 2:39

A firewall rule on the remote end? Assuming that the ip of your server is not being blocked by a firewall on their end, the remote admin should be able to run a packet capture on his\her SMTP server and see the incoming connection attempt and determine what's happening. They should also be able to look in their SMTP logs to see what's happening.

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From your description it sounds like the remote side is specifically blocking your Exchange server's IP address. This can happen if that IP is on a blacklist somewhere. We've had it happen a few times to our stuff, and once we got off of whatever blacklist we were on mail started flowing.

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