Can't send emails to some domains

Microsoft Windows small business server...
March 11, 2010 at 21:54:04
Specs: Windows 7, 3.5GB Ram
Up until about 1PM today, everything related to our Exchange server was working fine. At about 1PM, emails to external domains stopped going through. If I try to send an email to an internal user, it works fine. If I try to send an email to a @gmail.com address, it goes through. If I try to send an email to any other domain, it never sends. I can receive emails from all domains.

When the emails don't go, I can see them sitting in the Queue on the Exchange server. Again, the gmail.com queue has no problems, but all the other ones show a state of "Retry" and under "additional queue information" say "An SMTP protocol error occurred."

Any ideas where to start troubleshooting this?

-Ryan Adams

Free Computer Tips and more:http://RyanTAdams.com
Paid Tech Support: Black Diamond


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#1
March 11, 2010 at 22:40:17
Attempting to telnet to one of the mail servers we can't send mail to provided me with a tip: Apparently our Exchange server is black listed (http://cbl.abuseat.org/lookup.cgi?ip=63.224.131.126) for hosting a spam sending trojan. I figure this is probably the root cause of the issue.

I am currently in the process of checking the Symantec logs to see if any such trojans were recently detected.

On a related not, only our exchange server (at 192.168.0.2) should be sending out emails. How do I configure the PIX 501 firewall to block port 25 outbound for all other computers?

-Ryan Adams

Free Computer Tips and more:http://RyanTAdams.com
Paid Tech Support: Black Diamond


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#2
March 15, 2010 at 07:18:41
Create a rule on the firewall that denys all other IP address's other than the exchange from sending mail. Do check though that you dont have an authenticated relay. The easiest way to circumvent this is to get all users to change their passwords and to ideally use "complex" passwords. Just a side note i assume you have a PTR record for your domain name in public DNS as that would be the only other reason as to why you can send mail to Gmail. Generally as a rule all the major service providers will only accept mail from mail servers that have a reverse lookup. If they dont you may get grey or even blacklisted.

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