I already edited the main.cf file like this:
message_size_limit = 25600000 service postfix restart postconf message_size_limit message_size_limit = 25600000
I tried to send a mail with 18GB of total attachments and the log still showed:
NOQUEUE: reject: MAIL 552 5.3.4 Message size exceeds fixed limit; proto=ESMTP
Does something else can cause this?