By default, Mountain Lion Server’s installation of Postfix (the mail server), each email has a default size limit of 10 MB. That’s generally OK, but to avoid errors and undelivered emails, many people advise increasing the limit.
In order to increase the size limit to 25 MB, open Terminal. Type in the following command, then press enter. Once you press enter, you’ll be prompted to enter your administrator password.
sudo serveradmin settings mail:postfix:message_size_limit = 26214400
Although generally not required, you can restart the mail server. Do to this, open Terminal and execute the following command:
sudo postfix reload
Then, open the Server app and toggle the Mail service off and on again.
If you have any questions or trouble with these commands, just comment here and I’ll be happy to offer some assistance.