postfix - mailbox size limit and message size limit

Open /etc/postfix/main.cf with a text editor of choice and find message_size_limit directive and change accordingly. If it is not there, add the directive like this:

message_size_limit = 20480000

This sets limit to 20MB.

Some other parameters you need to change are in file php.ini which is usually located in dir /etc. Set their parameters as above or higher values as below:

post_max_size = 20M
upload_max_filesize = 20M

reload or restart postfix when you're done:

service postfix reload

or

service postfix restart