When you introduce a the first Exchange Server 2007 server into an existing Exchange Server 2003 organization you may encounter an issue with mail flow between the two servers.

Emails sent from Exchange 2007 to Exchange 2003 work fine, however emails sent from Exchange 2003 to Exchange 2007 do not work and can be seen in the deferred delivery queue of the Exchange 2003 server.

This issue can be caused by the use of a smart host configured on the Default SMTP Virtual Server on the Exchange 2003 server.


To resolve the issue remove the smart host from the Default SMTP Virtual Server and instead configure a new SMTP Connector for the Routing Group to send internet email (the namespace of “*”) to the smart host.

About the Author

Paul Cunningham

Paul is a former Microsoft MVP for Office Apps and Services. He works as a consultant, writer, and trainer specializing in Office 365 and Exchange Server. Paul no longer writes for Practical365.com.


  1. Cory

    6yr old post and still saved my ass! Thanks!

  2. 2k3-2010

    Hi Paul,
    I have a similar scenario and was wondering if this applies to my situation. I have a 2k3 front end and backend that are configured to forward to a smart host (Edge Transport) we are in the process of migrating to Exchange 2010 and have found that mail from 2003 destined for 2010 mailboxes are not being routed across the Routing Group Connector but to the edge transport which then sits in the Queue with a local loop error.
    Routing works from 2010 to 2003 but not from 2003 to 2010.

    In 2k3 the Default SMTP Connector is forwarding to a smart host.

    Your assistance would be much appreciated.


    1. Avatar photo
      Paul Cunningham

      I wouldn’t do it that way. Get rid of the route from 2003 -> Edge. Let outbound mail from 2003 go via the RGC then out via Ex2010 -> Edge.

      1. 2k3-2010

        Excellent thanks mate.

  3. Sameer

    Hi there,

    After doing this, i am still not able to send email to exchange 2010 mailbox from exchange 2003, please help in this issue

    I am able to send mails from 2010 to 2003 and towards Internet
    I am able to send mails from 2010 to 2010 mailboxes

    The only issue is from 2003 to 2010 mail flow


  4. Darren

    Thanks for the tip. This also works for exchange 2003 to 2010 migrations/updrages. I had the exact problem you described after moving test accounts from the old 2003 to the new 2010 servers. Cheers!

  5. Ekpo

    You are the best, man! I couldn’t migrate because of this issue….can’t thank enough.

  6. Peter Watson

    Thanks for this tip, solved my problem in an instant!!! Woul;d also point out that this was on a 2003 to 2010 migration, so the tip works for that scenario as well.

    Now if all goes well and I can move the mailboxes across overnight, I’ll get a day off in the sun tomorrow! So double thanks!

  7. igcas

    Thank you very much…it workd for me.

  8. Tom

    Thanks Paul, this posting helped me solve a Ex2003 to Ex2003 delivery problem. We added a second Ex2003 server to our setup last night but couldn’t get the messages flowing between them and out to our smarthost at the same time. Thanks again!

  9. Avatar photo
    Paul Cunningham

    DFad, on the Exchange 2007 server run Get-RoutingGroupConnector from the Exchange Management Shell and make sure you have RGC’s going both ways.

  10. DFad

    I have this same issue, where Exchange 2003 cannot send to 2007, but I do not have a smarthost. Any thoughts?

  11. TJ

    Thanks from an Exchange guy in Victoria. I already suspected it was that but it’s good to get confirmation before I do the configuration changes.

  12. saeed

    Thanks alott, I was experieencing this problem after exchange server 2007 installation, I am vegreatfull to you it realy solved my isuue, keep the good work continue

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