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SMTP error 550

We just upgraded our Exchange server to 2003 from 5.5 (hardware migration, also went from windows 2000 to 2003). Since the migration I've been having problems sending to Earthlink email accounts. Before we enabled external email for users (last July) Earthlink hosted our email, and they still host our web site. We have a different ISP who hosts our Reverse DNS zone, but Earthlink still has our DNS records and MX records. Here is the return message:

There was a SMTP communication problem with the recipient's email server.  Please contact your system administrator.
            <server2.colby.biz #5.5.0 smtp;550-EarthLink does not recognize your computer ( as connecting from an EarthLink connection.  If this is in error, please contact technical support.>

So I called tech support and here was my response:

550-EarthLink does not recognize your computer ([IP]) as connecting from an EarthLink connection.  If this is in error, please contact technical support.

..is due to an improper MTA configuration with the host that is responsible for delivering outbound mail on your network.  In a nutshell, the mailserver which attempted to deliver mail to the earthlink.net e-mail address was initially unable to successfully deliver the mail to the earthlink.net MX (more than likely due to high load on our end; no EarthLink MX host was available to accept the transaction at the moment of the delivery attempt), and so the EarthLink 'A' record was attempted by the sending mailserver.  The earthlink.net 'A' record, however, forwards port 25 connections to our outbound SMTP servers.  Unless the mailserver attempting the transaction maintains IP connectivity through the EarthLink network, delivery attempts through the 'A' record will consequently fail and the above quoted error message will be returned.

The behavior exhibited by the sending mailserver, in this case, is not standard.  According to RFC 2821, "Address Resolution and Mail Handling":

  "If one or more MX RRs are found for a given
   name, SMTP systems MUST NOT utilize any A RRs associated with that
   name unless they are located using the MX RRs; the "implicit MX" rule
   above applies only if there are no MX records present.  If MX records
   are present, but none of them are usable, this situation MUST be
   reported as an error."

Currently the EarthLink.net mx record resolves as follows:


You may wish to verify that you are able to successfully resolve this record through your nameservers, and that you can successfully route to these hosts.  Our engineers have informed us that if the sending mailserver cannot immediately establish a connection to deliver the intended e-mail, the MTA should attempt to retry a connection to the EarthLink MX, rather than defaulting to the earthlink.net 'A' record.

I also have slow response sending to comcast, aol and yahoo on some occations, but not all. Does this make sense? I thought that Earthlinks servers may have cached data and think that we were still hosted on their server, but they assure me that this is not the case. My DNS server looks fine and I don't have any connection problems internally or to the internet.

If this makes sense I have no idea how to remedy it. Can someone please point me in a good direction?

For reference, this is sending from my internal account to a users earthlink account, and I've telneted and verified SMTP connectivity on the server and the PIX box.
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1 Solution
These companies (earthlink, aol, and comcast) all uses some kind of blocklist filters.  One of the filtering rule they use (at least earthlink and aol) is that they check to see if you belong to a block of IP addresses that also belongs to another ISP.  For example, since I am using comcast's dynamic IP, anything I send out to those accounts will be rejected by them... Unless I relay my email through them, which they can identify as a "legitimate" email provider.

I am not sure if this is related, but all of the names you've mentioned (earthlink, aol, and comcast) all gave me a headeache when I try to resolve this problem.  What I ended up doing at the end, was to create a virtual SMTP gateway just for these name spaces, that relay the email through Comcast.  Could it be possible that you can do the same with XO?  perhaps that might resolve your problem.

Once again, this is just a guess based on experience dealing with email abnormality with these companies.  For all I know this might not even be related to what you are experiencing.....

- Info
David WilhoitSenior Consultant, ExchangeCommented:
is Exchange using internal DNS in its IP properties? Do you have any DNS entries on the SMTP VS itself?

ndegregorioAuthor Commented:
infotrader, I will try that tonight and see how it goes.

Kidego, Exchange is using external DNS and those servers are also listed in the SMTP VS.
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David WilhoitSenior Consultant, ExchangeCommented:
It shouldn't be using external DNS at all. Exchange needs the AD, and external DNS doesn't know about your AD. Take the external DNS entries off of the SMTP VS, and replace the entries in your IP stack with your AD's DNS server.

ndegregorioAuthor Commented:
Kidego, misread your question. Exchange server uses loopback, then internal (same thing), then external dns in IP properties. Under SMTP VS DNS nothing is listed. Added internal DNS there but still get bounce backs.
ndegregorioAuthor Commented:
Sorry, forgot about this one. I got it fixed and awarded the points. Thanks Infotrader.
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