Q: How do I adjust message size limits in Mail

I regularly send very large emails using Apple mail, typically 20 - 40 MB, but these get hung up in Apple Mail.  I understand there is a default setting of 10 MB in Mail.  I came across this article and have been trying to run these commands in Terminal.

http://support.apple.com/kb/HT5627?viewlocale=en_US&locale=en_US

However when I do I keep getting the following error message.

"User"-MacBook-Air-2:~ "Username"$ sudo serveradmin settings mail:postfix:message_size_limit

Password:

sudo: serveradmin: command not found

I've tried troubleshooting this but am not having any luck.

Any help and recommendations would be appreciated.

Thanks!

Kevin