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We have a server 2008 R2 with exchange 2010 sp1.

We have recently encountered a very wierd issue. When sending emails to a specific email userA@domain.com, the email is not received by the recepient, nor a error is returned. we received all emails sent by userA@domain.com, however we are not able to reply (emails simply do not reach)

On the other hand, sending emails to userB@domain.com, works fine

We have checked the issue with out ISP, which is the same ISP hosting the @domain, who claims to be a server problems.

After several checks on their side, they claim the mail request sent to userA@domain.com does not reach their MX server.

Any help is greatly apreciated

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Do your Exchange logs confirm that the mail was sent? Or rejected? Or processed? What are your logs saying at all about that issue? – mailq Aug 16 '11 at 15:45
Exchange Tracking center confirms the message was delivered to a another mail server. the message is beyond tracking point. This is the exact same message i get when tracking emails to userB@domain.com which are successfully delivered – Bdevelle Aug 17 '11 at 6:56
So when the Server (reached by the Exchange) is not under your control, then it is not your responsibility. Your server is working fine then. Push these server's administrator to track the mail to the next hop. – mailq Aug 17 '11 at 7:52
Indeed, however, due to the fact the server is using default SMTP connector, we do not know which mail server is the next hop. is there any way for me to track which mail server receives the message? or the only way is to configure ISP SMTP connector ? – Bdevelle Aug 17 '11 at 8:03
Huh? If you don't specify a relay host which delivers all mails on behalf of you, then the mails go to the MX of the recipient. If no MX is present then to the machine with the A record of the recipient's domain. This is only under the control of the recipient. – mailq Aug 17 '11 at 8:44
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You probably want to start with the Exchange console and message tracking to see if it can reveal where the "flow" stopped, if within your Exchange infrastructure. You might also check any intermediate queues (maybe in front of your Exchange) and/or filters that may be in place as well. Likewise on the remote/destination end too, if possible.

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Message tracking points the fact the email has left the exchange server for another mail server and is beyond tracking. I used Telnet to connect to the ISP mail server, sending email from within the mail server works fine. we have no filters enabled, and the only thing in front of the exchange is a CISCO 877 firewall, however i do not see why it would block a specific address only. The message does not arrive at remote destination (ISP MX server), only the message sent to userA@domain.com is having this problem – Bdevelle Aug 17 '11 at 6:41
You can determine the "next hop" as well as all subsequent "hops" by going through the headers of the non-affected emails to the same destination organization. Have them forward you the header(s) of non-affected email(s) that were delivered successfully. – user48838 Aug 17 '11 at 15:23
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