Email Queue is moving very slowly.

My Exchange Server (Installed on SBS 2003) has become extremely clogged up due to a continually occurring error:

Event Type: Warning
Event Source: MSExchangeTransport
Event Category: SMTP Protocol
Event ID: 7002
Date: 4/22/2010
Time: 9:19:07 AM
User: N/A
Computer: ######

This is an SMTP protocol warning log for virtual server ID 1, connection #4503. The remote host "", responded to the SMTP command "rcpt" with "451 Requested action aborted: local error in processing  ". The full command sent was "RCPT TO:<>  ".  This may cause the connection to fail.

This error seems to occur at least 3 times per minute. I've tried changing the network card, resetting DNS, Exchange, SMTP, POP3, Active Directory, the server itself and the stores. I'm able to telnet to the server, perform an NSlookup and do all of the basic Network troubleshooting stuff (Ping, TelnNEt, NSLookup, TraceRt, ect). The reverse-DNS lookups are setup correctly. This issue has been plaguing it for some time now, and we need a solution. Any help or suggestions anyone can provide would be helpful.

Emails are going out, but slowly. At this very moment I have 355 emails to one address in the Queue. In a perhaps related issue, we have a mail-enabled Public Folder, which is rejecting MOST, but not all incoming emails.
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SteveIT ManagerCommented:
Check with your ISP and check any filtering solutions you have in place

Also i'd check your DNS and also delete any mail that appears stuck, asking the user to resend.

Patch the exchange server

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As you said you are seeing 355 emails sent to one email address. Can you check if its valid email or send from application.  If its not a important email then try to clean and see if mail flow is restores.

try to study the mail flow happening in the orginization

dmoltrupAuthor Commented:
They are important emails, and are being sent to an accurate address. They flow through extremely slowly, but usually finally make it. We catch up a little on the weekends, but our daily throughput drives it up into hundreds in the queue.
SteveIT ManagerCommented:
Is your internet line working OK (speed wise) ? - do you have another line you can shift the mail queue onto  

e.g. when we had a problem with our main line, i setup ICS on a laptop with a 3G dongle to get the mail moving..
dmoltrupAuthor Commented:
There were two different issues here. After digging deep, I discovered the emails that were being queued were just from two users. These users' names were spelled wrong when originally created, and then when their names were corrected, all of the fields weren't checked in AD. This left some entries with the incorrectly spelled names. Apparently the email system was having trouble authenticating these users as a result.

As for the incoming email, somehow our firewall dropped the proxy. It was restored, and email started flowing in within seconds!
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