Client Session Could Not Connect To

I have a mail server that for the last two days has been getting the error:

Client Session Could Not Connect To 'mail server name'

Outgoing mails are all failing to send. There is no reason.

My mailserver is not blacklisted. It has a good rating.

My firewall has not had any changes to port 25. Everything for that box and that port is the same. I have made no changes to the  mail server at all. In fact, I even tried a recovery to last  month's backup of the mail server, just in case.

I run a test within the mail server and it sends me a test message fine, and everything passes.

I can't imagine why it is failing. All my sites are working, my internet is working, etc., so I know my firewall isn't broken.

Any ideas? It has been failing to send for two days.

The only thing done two days ago was adding another server to the network. I shut it down. Still mail doesn't send.

I had added settings for it to the firewall, but none to do with the mail server box. I even deleted all those settings and still won't send.

I have rebooted. I have reset the server. I have searched google. I have reset the mail server confg from last month's backup in case it got corrupted. I have removed  new settings I added to the firewall.

I now have 9000 mails in my outgoing retry queue. Please help.
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Starr DuskkASP.NET VB.NET DeveloperAsked:
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chicka616Commented:
Check that your gateway (router or external firewall) is not blocking SMTP.
Also verify your DNS settings and that they match the original setting before you made any changes.
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Starr DuskkASP.NET VB.NET DeveloperAuthor Commented:
my firewall has smtp port 25 open.
I didn't alter the DNS setting.
The only thing I can think is perhaps my dns server through AT&T might have had someone go in and change the smtp pointer. Could that be an issue?
Maybe  they were changing someone else's pointer and changed mine? Would that cause this kind of problem? Is there anything my service provider could do on their end to cause this?
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chicka616Commented:
That is possible.
I once had my IP range blacklisted, but speaking with my ISP and getting them to troubleshoot from their end also helped.
Check that the ISP did not block port 25 by accident or changed the SMTP pointer.
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Starr DuskkASP.NET VB.NET DeveloperAuthor Commented:

Ok, this is fixed.
Here's the scoop:
We purchase another identical netgear firewall to what we have. Installed it, updated the firmware, reloaded the config file, and it failed too.
So we reset it to factory settings, updated the firmware and manually put in all of our config settings. That did it. It was apparentl a bad config file.
I had even used a backup version from over a month ago, and still it was bad. So go figure. It didn't break down a month ago, only a week ago, so why the month old config file failed too, who knows. But all is well now.
Thanks all!
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