Receiving Error 553 Message Rejected, Host IP Address Does Not Resolve

I sm running Exchnage 2000 on Windows 2000 Server SP4, when we receive e-mail from some, not all, users and attempt to reply we get the following error message:

The following recipient(s) could not be reached:

      'name@domain.com' on 3/27/2006 11:49 AM
            There was a SMTP communication problem with the recipient's email server.  Please contact your system administrator.
            <mail.fthcm.org #5.5.0 smtp;553 message rejected: your host's IP address does not resolve: 68.163.94.178>


Can I get some assistance, please.. We do have an internal DNS server that resolves that IP Address, however, the IP Address is the IP of the ISA Server and not the actual internal e-mail server.  Thanks..
fthadminAsked:
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SembeeCommented:
You are probably getting caught in reverse DNS lookups. If your email goes out through the ISA server then you need to configure a DNS lookup, reverse DNS lookup and what the server announces itself as. Without those values being set you will fine more and more domains will refuse your connection as an antispam measure.

Simon.
fthadminAuthor Commented:
Should the reverse DNS entry be added to my internal DNS Server?  Or is this something that our ISP has to add?  Who is actually rejecting the message?  I thought it was the mail server of the domain that the mail is being sent to??
SembeeCommented:
The reverse DNS is set by your ISP. Don't let them fob you off to the domain name registrar, as reverse DNS is set on the IP address.
The email is being rejected by the server you are trying to send the message to, because as part of its antispam defense it is checking whether you have a reverse DNS.

http://www.amset.info/exchange/dnsconfig.asp

Simon.

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