In Exchange 2003 some of the default settings were changed to tighten security. One of these settings is the sending and receiving message size settings. In Exchange 2000, no default size was set, but in Exchange 2003 the Sending message size and Receiving message size settings for the Exchange organization are set to 10240 KB. This default setting applies to new installations, or upgrades from Exchange 2000 in which no default size was set.
During an upgrade if you have customized these settings, the custom settings are retained. However, the default settings are overwritten. So, if you do not want a sending and receiving message limit imposed on your users, you need to manually change the defaults after an upgrade. If a custom default size is already specified for the organization, the existing setting is preserved.
To change the default sending and receiving limits:
In Exchange System Manager, expand your Exchange organization > expand Global Settings > right-click Message Delivery Properties.