No users are getting any email, senders to the school are receiving an undeliverable mail response from the Exchange server.
On the server event logs for check for multiple errors for each email:
Event Type: Error
Event Source: MSExchangeTransport
Event Category: NDR
Event ID: 3020
Description: A non-delivery report with a status code of 5.4.6 was generated for recipient x-ex;/O=OrgnizationName/OU=AGName/CN=RECIPIENTS/CN=recipientName (Message-ID <ID@ServerFQDN>).
Cause: A forward loop was detected by the categorizer. This is a common hosting configuration problem caused when someone uses the provisioning tool to create a contact in one organization unit and creates a user in a different organization user that share the same e-mail address.
Solution: Verify that you do not have a user in organizational unit and a contact in a different organizational unit that have the same e-mail address.
Check AD Users and Computers make sure – no users have the same email address
Should this not be the case it could be the ADSynch utility that is used for syncing the local accounts with Office 365 updates the ‘targertAddress’ property in AD for each user account – this is so that local mail can be redirected to the Office 365 email address.
Unfortunately, because the email domain iscacollege.devon.sch.uk was still hosted on the local Exchange server, this had the result of the email coming into Exchange, the Exchange server forwarding it on to the targetAddress value, which in turn sent the email to the Exchange server . . . and so got itself into a loopback situation.
Although you can view the targetAddress property in AD Users & Computers (through adding it as an additional column under the View menu) you can’t actually edit it from the user properties, so you have to run ADSIEdit to clear the setting – once that is done, the email doesn’t loop and Exchange works again.
ADSIEdit is fine for single user edits, but we had about 700 users to fix, but luckily there’s a handy tool called ADModify (admodify.codeplex.com) that works brilliantly (and quickly) to do that.
So if anyone else suddenly has their Exchange server die shortly after syncing with Office 365, check the targetAddress property for each user in AD!
Reviewed on 25/10/2016