I opened a case with microsoft for this error - the issue for me was that the docs (both on splunkbase and in the Microsoft docs article referenced by Splunkbase) say that the Message Trace API can gather data up to 30 days prior. This is incorrect. The correct documentation can be found here, which shows that only a max of 10 days prior is allowed: https://docs.microsoft.com/en-us/powershell/module/exchange/get-messagetrace?redirectedfrom=MSDN&view=exchange-ps#description Switching the start_date from 30 days to 9 or 10 days prior ended up working for me!
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