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DNS and Exchange 2003

I have a few question concerning DNS, ,and exchange 2003. Users on my network send an email message to colleague's who have valid email accounts sometimes the email comes back right away, and something its weeks before they get an error like the one below.

            "The e-mail system was unable to deliver the message, but did not report a specific reason.  Check the address and try again.  If it still fails, contact your system administrator.
            <xxx.xxx.com#4.0.0 smtp;451 <jwiv@uga.edu>... Requested mail action not taken: mailbox unavailable>"

Also what are the benefits of using reverse DNS? and should I be using it?

Also has anyone run into a problem with the greylisting?

Any information would bee greatly appreciated.
3 Solutions
You need to ensure that you have a reverse lookup entry that matches your outbound mail IP address.  Many companies and universities use it to validate whether you are who you say you are.  This prevents people being able to send mail to other organizations as Microsoft.com or send SPAM.
Neadom TuckerCommented:
Yes Reverse DNS Lookup is REQUIRED in todays mail server environment.  You also need to make sure that you have SPF records created.  Many spam and mail servers look for SPF records to insure the mail server that is sending messages has rights to send messages.

You should check your DNS at dnsstuff.com it will run through the list and tell you what issues you may have with your domain.
Is this when they are sending to someone on your local exchange serer?  or is when they are sending to someone offsite.

It's common to have this problem when you do not make a reverse setting on your dns it's very easy to add just open your DNS Manager then go to reverse Lookup Zones.  Then right click add zone.  Then the wizard will start press next,  you want it as your primary zone and make sure active directory box is checked, then press next, you will want to make it to all domain controllers in the active directory, then press next,   type in the internal first three octet of the ip block you have 192.168.1 (or whatever ip is)  press next till it finished and now your set.  Dns does not setup this by default you have to add it when you set it up.  This will even improve network performance and make dns work right...  

More help look at this article

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