As an Exchange 2010 SP1 administrator, you might be forced to increase the limits imposed by Exchange to the size of the messages it can handle. Those limits ensure, among other things, that the server will not easily be the victim of DoS attacks due to over-sized messages.
One place where you can modify those limits (you should modify other limits actually, like connectors limits), is in EMC at Organization Configuration > Hub Transport > Global Settings > Transport Settings:
According to the help page of the dialog, if you clear the check boxes of Maximum receive/send size (KB), you configure your server with no limits at the Organization level. This sets the configuration for the transport as “unlimited”. There’s a little trick here and it’s related with OWA. If you go now to OWA and try attaching a “big” file, say, 25 MB, OWA will complain saying:
“The following files weren’t attached because they exceed the maximum size limit for attachments (5 MB)”
AFAIK, OWA has a default limit of 35 MB (which is less than our 25 MB file). Well, that’s correct as far as you don’t set the Organization level transport limit to “unlimited”. This makes OWA to believe that “there are not limits configured” instead of “there are no limits”; given this, OWA imposes a 5 MB limit. To solve the problem, simply specify a value to the limits shown in the dialog window above. The maximum value is 2097151 (almost 2 GB) which I think it’s plenty for anyone around.