Could not send txt attachment via mail

Hi

For business requirement, I need to generate the invoice information and save it to FTP server, then send the file to user via mail. So first I use a program to generate the invoice file and then use  shell script to send mail with attachment. But when i send attachment which format is txt, once the attachment bigger than 1kb, the system could not send mail. But it's fine for CSV or some other format attachment. The  directory which the file stored has enough privilege  to write/execute/read. So i'm guessing is it possible that the server has some setting need