:: 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

postconf | grep message_size_limit
# default is 10MB
message_size_limit = 10240000

+ edit main.conf on this line

postconf -e "message_size_limit = 5240000"

+ checking new configuration

postconf | grep message_size_limit
# change to 5MB
message_size_limit = 5240000

+ restarting postif

/etc/init.d/postfix restart

 
:: Link
+ Google
+ Janjonas