In any case, the setting MaxMessageSize, which has a default of 10 MB in Exchange 2007, is the parameter that needs adjustment. If you want to get a look at the values for the MaxMessageSize parameter for all of the SMTP receive connectors in your Exchange organization, start up the command line-based Exchange Management Shell and issue the following command:
get-receiveconnector | select identity,maxmessagesize
Now, to increase this to something larger–say 30 MB–use the set-receiveconnector, as in the following command:
set-receiveconnector “Connector Name” -maxmessagesize 30MB
As I mentioned before, make sure to run this against the right receive connector