Exchange - Changing email adresses - Could not deliver to recipients address - Error 5.00

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http://serverfault.com – The company I'm working for previously bought a small company who had their own e-mail service via some provider. And now when we're trying to merge them to our domain, we get this error message when we try to send an email to the old address via gmail for example. What we've done is this: - Add the old email address as an alias in our Exchange - Disclose the email service form the old email provider When we send an email to the old address via our own Exchange, the email delivers just fine. Just not externally, via gmail, yahoo, hotmail etc. What is the cause of this? Could it just be a D (HowTos)