Inbound email stopped working. (SBS08)

Today one of my clients inbound has stopped receiving email. They are getting either no emails or senders receive a bounce back.

Below i one of those bouncbacks.

From: System Administrator - Boston <>
Date: February 21, 2011 11:15:10 AM EST
To: Joseph Jennings <>
Subject: Delivery Final Failure Notice

Can not deliver the message you sent. Will not retry.

Sender: <>

The following addresses had delivery problems

<> : Reply from[]:
       <<< 500 5.3.3 Unrecognized command

Exchange Remote Connectivity Analyser says this...

      Testing inbound SMTP mail flow for domain
       ExRCA failed to test inbound SMTP mail flow.
      Test Steps
      Attempting to retrieve DNS MX records for domain
       One or more MX records were successfully retrieved from DNS.
      Additional Details
      Testing Mail Exchanger
       One or more SMTP tests failed for this Mail Exchanger.
      Test Steps
      Attempting to resolve the host name in DNS.
       The host name resolved successfully.
      Additional Details
      Testing TCP port 25 on host to ensure it's listening and open.
       The port was opened successfully.
      Additional Details
      Attempting to send a test e-mail message to using MX
       Delivery of the test message failed.
      Additional Details
       The server returned status code 500 - Syntax error, command unrecognized. The server response was: 5.3.3 Unrecognized command
Exception details:
Message: Syntax error, command unrecognized. The server response was: 5.3.3 Unrecognized command
Type: System.Net.Mail.SmtpException
Stack trace:
at System.Net.Mail.DataStopCommand.CheckResponse(SmtpStatusCode statusCode, String serverResponse)
at System.Net.Mail.SmtpConnection.OnClose(Object sender, EventArgs args)
at System.Net.ClosableStream.Close()
at System.Net.Mail.MailWriter.Close()
at System.Net.Mail.SmtpClient.Send(MailMessage message)
at Microsoft.Exchange.Tools.ExRca.Tests.SmtpMessageTest.PerformTestReally()

They are using Postini Spam filtering. I have since disabled this AND changed MX's back to point directly at their server, same thing.

This system has been working perfectly for months. This is the first time there has been an issue like this.

Please help.
Who is Participating?
Alan HardistyConnect With a Mentor Co-OwnerCommented:
MegaNuk3Connect With a Mentor Commented:
Also ensure that is resolving to the correct external IP address and your firewall/router is sending that traffic on port 25 to your exchange server
VCSLIAuthor Commented:
The only service i can find on SBS is the "Transport Service", not the edge transport service. I have restarted that however mail hasn't been restored.
Wonko_the_SaneConnect With a Mentor Commented:
Any changes to the firewall/virus scanner in front of Exchange? SMTP Syntax errors point that something is messing with the data stream, e.g.:;en-us;320027

VCSLIAuthor Commented:

I attempted to check the routers firewall rules... Low and behold i couldn't open the admin CP, page just wouldn't load.

Had one of their staff reboot it and now Postini is sending down all their mail.

Monday Monday..... Can't trust that day.....

Thank you all so much for your help. Points for everyone!
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