Size limits for email messages are controlled in a variety of places in Exchange 2007. Settings are placed globally at the transport level, on each individual send or receive connector, and at each individual mailbox. Your business needs should determine if you need to change these settings and at which location(s). In order to accomplish this successfully, you must know that more restrictive settings override less restrictive settings as you flow up the chain from individual mailbox settings to SMTP connectors and finally to global transport settings.
Global Transport Settings: Get-TransportConfig displays global transport settings for all Hub and Edge Transport servers in the Exchange Organization. By default, there is a 10MB size limit on send and receive. This will override all other less restrictive message size limit settings set elsewhere.
NOTE: Exchange 2007 considers message submission to the store driver as a “send” operation. Therefore, if you are attempting to receive an email message on an SBS 2008 server from the internet that is greater than the MaxSendSize of the global transport configuration, delivery will fail