From the Exchange Powershell run the following This command will display the current size
Get-ReceiveConnector | select identity, maxmessagesize
This will set the size to 50mb and the timout to 9 min.
Get-ReceiveConnector | Set-ReceiveConnector -MaxMessageSize 50mb -ConnectionInactivityTimeout 00:09:00
Then restart transport…
This will set all receive to 50 mb, use with caution!