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One of my clients received this error below while trying to attach an email to a case.
Reviewing the trace log I could see the error code was 29 and the issue was incoming email rejected.
Looking over the trace logs,, I find 74 instances of this error code over that last 3 months (from the 4 same users).
What the user did in the first instance that was brought to my attention was try to save an email that has not been sent yet (she saved it in outlook, and is waiting for a valid email to send the email). She then tried to attach the email to the case, causing this error.
Our current version is 2015, Online. The Outlook Client version is 126.96.36.19943.
We also have this problem in one of our Test environments. Email Synchronisation is server-side, and all Tests are going through with no error.
We too have this issue with one of our CRM systems. Did you open a ticket with MS?
Did you find a resolution for this?
Please refer the following community threads.