When migrating data it might be seen that items below this size limit still fail to ingest even the Exchange Administrators have implemented a maximum size in Exchange.
When messages are ingested there is an overhead in the packing of data to be sent to Exchange. This is described in the Microsoft articles:
Exchange 2007 and 2010:
“This setting applies only to Microsoft Exchange Server 2010. It indicates the maximum message size that is accepted by EWS. This maximum message size is 35,000,000 bytes, which translates to 25 MB of Base64-encoded data.”
Exchange 2013 and 2016:
In practice what this means is that the maximum message in Exchange needs to be artificially raised, or, it needs to be understood that messages below this limit may not ingest, due to the behavior of Exchange.