Emails and DNS

We are hosting a domain's email on a server we have that's on it's own WAN address.  We also host our companies website on it as well, while our email is on another server with it's own WAN address.  They are on different subnets as well.  They can send and receive emails from everyone, but they can't send one to us.  We can send and receive emails to/from everyone without a problem.  I know it's a DNS issue but I don't know where to begin to look.  Any help is appreciated.  Thanks.  
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gsmartinConnect With a Mentor Manager of ITCommented:
First, make sure both mail servers are resolving DNS, for each other’s domain, via the internet.   So verify the DNS servers and/or host files being used by each server to ensure no entries are preventing proper resolution.  This is necessary in order for the mail servers to resolve the domain of the email address, and retrieve the domain’s MX record.  The mail server will then be able to deliver mail to the referenced MX record’s DNS name (i.e. and vice versa.    As long as each mail server can equally resolve this information for the receiving mail server then mail should flow.  
Here are a couple of websites that will help you confirm the external DNS and MX record health of each domain.  These are very useful tools for this purpose.  FYI… With MXToolBox you can also check to see if a mail server has been put on a Blacklist.
Can you post an NDR , if you have access to one?
How do you know it is a DNS issue?  Does nslookup not give back the hostname?  Are they able to telnet to port 25 on your mail server?  Is there any internal routing or is this strictly over the internet?
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Provide NSlookup details for further analysis. Seems to be a local routing issue...
dsyncdAuthor Commented:
*** Can't find server name for the address Non-existent domain
Default Server: Unknown

It shouldn't be routing locally at all.  I'm getting that it's a dns issue from running smtpdiag.  I can post the whole thing if needed but this is what it errors on.  Changing domain name for privacy of course:

Warning: The TCP DNS query returned no results.
Checking MX records using UDP:
Error: No MX or A records were found for the remote domain. Verify that the
remote domain is valid. Your firewall allows outbound DNS queries (Windows
NT/2000 Server requires TCP), and your DNS server can resolve external domains.

PapertripCommented: is a private internal address -- why are you doing a lookup on that IP?  It doesn't appear to be the IP for, because smtpdiag says there is no A record for, so nslookup wouldn't give you the IP to do a lookup on like you did.

If this is supposed to be internal routing only, then check out this link for server 2k3 and this for 2k8 -- if you are running something else just Google for adding DNS records to whatever version you are running.
Ignore that nslookup result - it just means that there's no PTR record for and has nothing to do with this issue.

The smtpdiag results are interesting - did you run it from the other domain's mail server?  It's saying that it can't find host or MX records for the domain, which is presumably your domain.  Since you are able to receive mail from everyone else except for that one other domain's server, you obviously do have the correct host and MX records for your domain in the public DNS.  So the problem must be that the other domain's server can't find them for some reason.  Perhaps it's using a DNS server with a bogus zone on it or something like that.

An NDR would probably be very helpful.
dsyncdAuthor Commented:
The e-mail account does not exist at the organization this message was sent to.  Check the e-mail address, or contact the recipient directly to find out the correct address.
That doesn't sound like a DNS problem; it sounds more like a mistyped address.  Can you telnet to port 25 on your server from the other server and send a message using SMTP commands?
I am wondering if the Exchange that can not deliver to you has a recipient policy for your SMTP domain name, after all you said it does host your website so maybe the active directory domain name is the same as your smtp domain name, a contiguous namespace?

If it's exchange 2003 check the recipient policy, if it's exchange 2007/2010 Check accepted domains.
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