In Exchange 2003 it was very easy to change the message size limits – each user had their own settings. 2007 has a little more difficult process, and it groups everyone as one. We’re currently not using SP1, so some of the steps will probably change. SP1 will eventually make its way on our servers, but until then we’ll just stick with a patched standard version.

To change the maximum allowed OUTSIDE your domain (outgoing email):
Set-Transportconfig -maxsendsize size_in_bytes
Obviously replace size_in_bytes with the size you require.

To change the maximum allowed INSIDE your domain (incoming email):
First, we need to find the Identity of your server (or servers)
Get-sendconnector | List
You’ll find a bunch of information printed on your screen, we only really need the MaxMessageSize and the Identity. If the MaxMessageSize is set to something you already like, you do not need to change anything.
If you have more than one server in your environment, you must do this command for each of the servers.
Set-Sendconnector -Identity identity_name -MaxMessageSize message_size_in_bytes
Replace identity_name with the identity of the server and message_size_in_bytes to the actual size you want.