Delivery has failed to these recipients or groups:
This message is too large to send. To be able to send it, make the message smaller, for example, by removing attachments.
The fix to get this to work is to execute the following commands in Exchange command:
Set-TransportConfig -MaxReceiveSize (X)MB -MaxSendSize (X)MB
IE: Set-TransportConfig -MaxReceiveSize 30MB -MaxSendSize 30MB
In my situation I did not have to restart any services, just simply resent the previously denied email and the client received it.
My reference site:
I am in no way responsible for the actions you take with the information provided. Please note that this worked on my Exchange 2013 server but not necessarily with all servers. I provide this information on an educational basis and from educated experience only. If you are unsure about if this will work in your situation please continue researching it.