Messages not getting to internal users from internal users

We have had some instances were a user A sends user B an email, both are internal exchange accounts.  User B does not get the email.  User B rants and raves and calls user A who sends the email again and user B now gets it.

I have done the following to try and discover the intermittent problem.

Checked user A sent items and outbox, the email is NOT in the outbox, and both copies, the original and the resent email are in the sent items.
Checked user B inbox, Junk and Deleted folders.  Nothing for the first try, yet the second attempt landed in the Inbox as expected.
Checked the message for Spam triggers, nothing
Checked the Anti-Spam and Anti-Virus logs, nothing
I tried to track the messages from User A to User B by using the message tracker in Exchange, message 1 does not show up at all, message 2 does all the expected steps. leaves User A gets to User b.
Checked the firewill logs to see if something there blocked it the first time, nothing.

I'm wondering if anyone has experienced this and how to prevent it from happening again.  Are there any tools out there that can help us determine what is going on?  

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mcioffi209Connect With a Mentor Author Commented:
Figured it out.

Turns out that there was an old POP3 account that was running ahead of the exchange account.  For some reason Outlook was trying to send via the POP3 from time to time but not all the time.

This issue can be closed.
Is this happening with just some specific users, or randomly with any combination of users?  What version of:
Windows Server
Service packs

Exchange relies on AD and DNS being properly configured.  If you have installed Exchange 2003, the AD schema needs to be properly updated. Are you seeing errors in your event logs? What Exchange services are running?
 Sorry for all the questions, but a lot more information is needed here.

mcioffi209Author Commented:
I understand the need for additional information.

AD should be fine, DNS may be to blame here as we had some DNS issues when the firewall crashed.

I will have to check on the service packs and get back to you.

We are running Small Business Server 2003 with exchange 2003 server.

By the way, what is a best practice method of cleaning up DNS?  We have a bunch of old bad records.

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mcioffi209Author Commented:
what would you use to generate a list of patches installed?  I'm not sure how much detail you feel necessary.  

So far:
Exchange: 2003
Windows Server: Small Business Server 2003 Standard.
Service Packs: It has SP3 but not a lot of smaller packs  Do you have a tool that will create a list that I can cut and paste from?

Glad you found the solution. It's often the little things that we overlook that caus the most problems.
Closed, 500 points refunded.
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