:: What is Postfix
Postfix is a free and open-source mail transfer agent (MTA) that routes and delivers electronic mail. It is intended as a fast, easier-to-administer, and secure alternative to the widely-used Sendmail MTA. – WikiPedia
:: How to setup attachment size limit
+ checking limit size for attachment

+ edit main.conf on this line

+ checking new configuration

+ restarting postif

:: Link
+ Google
+ Janjonas