By default Exchange 2007 with Service Pack 1 has a number of limits set that restricts the size of messages that can be received to 10MB.
To see what the current limit is open the Exchange Management Shell (found in Start->All Programs->Microsoft Exchange 2007) and enter the command: get-transportconfig
This will show you a list of settings and the current limits, for example MaxReceiveSize. To change a value you can type the command: set-transportconfig -maxreceivesize 30
That would set the MaxReceiveSize to 30MB.
You may also need to tcheck to properties of the receive connectors in the Exchange MMC Snap in. Go to Exchange -> Server Configuration – Hub Transport and view the properties of one or each of the connectors. On the general tab is a setting for the maximum message size.
In addition to the checking the setting of the Internet send connector may solve any problems. This is found under Exchange -> Organisation Configuration -> Hub Transport. Again view the properties of the connector to see the Maximum Message Size setting.