The connection was dropped due to an SMTP protocol event sink

Have a situation with a client right now where the mail queue is stacking up on a sbs 2003 r2 server.  when viewing the queue i can see the following error:

The connection was dropped due to an SMTP protocol event sink.

Ive tried restarting the exchange server and firewall.  I can receive email no problem but am unable to send.  DNS appears to be working correctly and a DNS report does not spit back any red flags.

Any Suggestions?
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breynolds01Connect With a Mentor Author Commented:
It turned out being an endian  software firewall that was on. It seems that the sbs server was originally configured to pass traffic through the software firewall, but it has been having some issues as of late.  After shutting it down the queue slowly started to clear
Is the server running any kind of spam filtering software?  I've seen an "SMTP protocol event sink" listed as the reason for failed SMTP communication on a server with Sunbelt Messaging Ninja on it before.
Philip ElderTechnical Architect - HA/Compute/StorageCommented:
Use Telnet to test an SMTP connection from the server's desktop to your ISP's SMTP servers:

If the connection works, then you are guaranteed that something is not choking SMTP.

On the Exchange side, make sure that the SBS SMTP Connector is setup correctly. Verify that it is sending via DNS.

You can run the CEICW to reseat the correct settings (SBS Console --> To Do --> Connect to the Internet). Leave the certificate alone and make sure that the services you want open to the Internet are selected during the wizard.

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