Using the MaxReceiveSize and MaxSendSize you are able to limit the messagesize that is allowed to be transported. You generaly want to do this to block huge (in SMTP world) messages. These huge messages (when received and send) might require a huga ammount of system resources slowing down the overall performance or even make the outlook client go unresponsive.
An exchange administrator is able to alter these limits which are by default;
Exchange 2007 RTM (out of the box media)
send : Unlimited
Receive : Unlimited
maxRecipientEnvelopeLimit : Unlimited
Exchange 2007 SP1
send : 10MB
Receive : 10MB
maxRecipientEnvelopeLimit : 5000
Using the Exchange Management Powershell console you may use the following commands to alter these limits.
View the actual limits.
Alter the limits
Set-TransportConfig -MaxReceiveSize 20MB Set-TransportConfig -MaxSendSize 20MB
If you want the full documentation please review the following Microsoft Documentation…