Why is some e-mail being returned un-deliverable?

I am running Exchange Server 2003 to handle my companies e-mail needs.  I am experiencing a problem with sending e-mail to certain addresses.  I can send e-mail to addresses like hotmail or yahoo or big ones like that, but i am trying to send e-mails to people that have smaller ISP's that offer e-mail service and they are being rejected somehow.  What could be causing this and how can I fix it so that we can communicate with all e-mail addresses and not just some?  I have checked to see if my server IP is blacklisted and it is not so that is not the issue.  I have attached the response that I get from the auotmated System Administrator.

Your message did not reach some or all of the intended recipients.

      Subject:      Test E-Mail
      Sent:      10/16/2008 1:31 PM

The following recipient(s) cannot be reached:

      'nocarb@intergate.com' on 10/18/2008 1:38 PM
            Could not deliver the message in the time limit specified.  Please retry or contact your administrator.
            <gracedc01.4grace.org #4.4.7>
gccITteamAsked:
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SirDragonCommented:
This could be caused a number of reasons, here are few ideas:

1.  The recieving company might be using reverse DNS lookups on your MX records.  You need to have an entry for your MX record at the registrar or whoever is holding your Zone Record.

2.  if you use a third party vendor or appliance to control spam, is your MX record pointing to that IP address, and if so, do you redirect outboud email to the same IP?  in other words are your Inbound and Outbound IP addresses the same.  If not you need to register both IP addresses in your Zone Record.  Again, on a reverse DNS lookup if the sending IP address is different from the register MX IP you might be rejected for SPAM.

3.  There is a new record called the SPF record.  Not very popular yet, but look into it.  It might make sense your case.

4.  Finally, some bigger companies, have weird "way" of performing MX record lookups and blacklisting of Server.  Verizon is notorious for this.  They have their own list for blacklisting servers and it's harder to get off of them.  

Hope these help!
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gccITteamAuthor Commented:
Where do I find my Zone Record?  We have our website hosted with a company and they use to handle our e-mail as well.  We have placed our MX Record there so that when people e-mail us it goes to them and they re-direct it to our static IP set up in our server.  How would I find out if my inbound and outbound IP addresses are the same?  I have done some research on the SPF and have already done that in the DNS of our server.
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dcsdaveCommented:
Do you have a reverse DNS entry setup with your ISP?  That solved a few problems for me with AOL and a few smaller ISPs.
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SirDragonCommented:
Ok since they are hosting your email, this means that all emails go to the hosting company, and then to your server.  When you send out... do relay emails to the hosting comapany and they send it out? or do they go from your server to the world?  
If option 2 is the case, it explains the problems you are having....

Your mx record points to the hosting company.... when you send out messages, the originating IP is your public IP address.  So when the recipient does gets the email, the header will have your IP address, they will try to resolve this IP by checking the MX record which points back to the hosting company.  they think you are spamming them and they drop the message.

The solution is to talk to your hosting company, and ask them if you can relay all messages through them.  Or make the appropriate entries to your Zone Record to add your piublic IP address.

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ifreqCommented:
There is anything strange on this case. Especially there is nothin wrong in your side. It is good to ask thought :-)  As you said when youre trying to send emails to smaller ISP's they usually get rejected.

Reason is simple:  Receiving organization  dont have enough expertise / resources to keep up working email-hosts. Usually the servers are old hardware, which is loaded to the top.  Smaller ISP's (not all) but few also use rented VSDL/ADSL lines which are not capable of handling huge amount of smtp-traffic.

There is nothing you can do for this really, maybe trying to convince your colleagues to change for better smtp-provider.

Trust me, ive been on this same business as operator over 5 years now :-)

Your NDR states the following on this case:

The following recipient(s) cannot be reached:

      'nocarb@intergate.com' on 10/18/2008 1:38 PM
            Could not deliver the message in the time limit specified.  Please retry or contact your administrator.
            <gracedc01.4grace.org #4.4.7>

Your Exchange has been trying to send email to the receiving host for VALUE time (usually 1-5 days).
The receiving server has not been responding in the given  time and you have got the NRD- message back to you explaining the situation.
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gccITteamAuthor Commented:
how do i make entries to my zone record so that i can add my static public IP to it?
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SirDragonCommented:
You zone record is usually handled by either the ISP, the Domain host or your Registrar.  Depending on how you have it setup, you might have to send the request in, or you can do it yourself online.

If you are not sure, do a nslooukup search online and look for the SOA, this will tell you who owns it!
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