Recipient deferred because there is no Mdb


  • We have a child domain set up in AD for external users to access our Sharepoint resources, as part of this their external email address is added to the 'proxyAddresses' attribute to allow email notifications, etc.

    We then find, for some of these accounts at least (doesn't appear to happen straight away), we can't email them normally through Outlook. Users receive the following message:

    '420 4.2.0 Recipient deferred because there is no Mdb'

    The Exchange system is obviously picking up the external address in the child domain and trying to deliver the message to it. These users do not have mailboxes within Exchange which is why the above message is being generated.

    Is there anything I can do to get Exchange to ignore the child domain? At present the only way to fix this seems to be to delete the AD account for this user so Exchange goes back to delivering the email as normal.

    Thanks for any help, this has been annoying me a couple of years now (we had this problem with Exchange 2010 as well, now fully migrated to 2013 CU10).

    Monday, January 25, 2016 12:25 PM


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