If you want to increase the maximum size an
e-mail can be that is accepted as inbound mail by Exchange it can be done using
the ECP, you must navigate to Mail Flow > Receive Connectors and use
the .... button to expose the Organization Transport Settings
option.
From the Limits tab, you can increase the Maximum
Receive Message Size (MB): I changed mine to 25 MB's, while I was in here I also changed the outbound limit to 25MB.
The following PowerShell command can be used to check the ECP has made the configuration change properly.
Get-TransportConfig | ft
MaxSendSize, MaxReceiveSize
|