Configure internal dns server to point to external mx records

I have an internal domain as an example co.com.  Co.com is a Windows 2008 domain and has an internal dns server.  I have applications on the internal network that need to send email to co.com.  The email server for co.com is hosted outside of the co.com network and domain.  I have the mx records setup and can send email to the co.com from outside but not from inside hosts of the co.com domain.  How do I make the internal DNS server for co.com send email traffic out to the mx records listed in external dns?
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dmwynneAsked:
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davealfordIt SupportCommented:
What is the mx setting for co.com on the internal dns server?
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dmwynneAuthor Commented:
Currently it is not set.  I tried setting it to the external mx records but it did not work.
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mattconroyCommented:
You will need a reverse lookup record(PTR) for your Exchange Server in the external DNS for one thing.
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davealfordIt SupportCommented:
You would only need a PTR IF your email host insisted on it. Most do. To test it, Try sending an email from a PC inside your network to the IP address of you mail host. You will then see any error message it is giving -
-----------------------------------------------
telnet 'ip address of mail host' 25
helo mail.co.com
mail from:administrator@co.com
rcpt to:administrator@co.com
data
subject:Test
Testing
.
-----------------------------------------------
If that doesn't work then it may be your mail host requires PTR. If it does work then all you should need is to add an MX record for co.com The MX record SHOULD be an A record NOT an IP address.
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kevinhsiehCommented:
Okay, your internal AD domain is co.com. Your email servers for co.com are external, and you have internal mail servers that need to be able to relay mail to co.com. You have an internal DNS zone for co.com. Do I have things correct?

If this is the case, you need to add MX records on your internal DNS zone that match the public DNS records, AND you also need to include the correct A records for hosts that are in co.com.

Example:

@ MX 50 mailbag.hostingprovider.com.
@ MX 40 mail2.co.com.
@ MX 20 smtp.co.com.
mail2 A 207.23.12.3
smtp A 207.23.6.5
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dmwynneAuthor Commented:
I added the mx records again and it seems to be working, not sure why it did not work the first time.
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