Postfix (https://de.postfix.org/httpmirror/) is my MTA of choice. I use it for my mailserver due to its simplicity , security and sendmail-compatible (the widely used smtp in the world but not as secure). It is also extensible by plugging other servers for various purposes (antispam, antivirus,database etc).
I had one problem with file attachment larger than 10MB. Users couldn’t send it although I have setup squirrelmail (SM) to be able to attach files summed up more than 20MB and I had modified php settings as already. The problem was not obviously not to be found in the SM setting. It was postfix. The default, attachment size that can be sent by postfix is 10MB ~ 10240000 byte. How did I know? I looked in the log file (for my system it is in /var/log/mail/errors. For other system, the file to look is /var/log/maillog). The line looked like this:
Feb 26 16:30:53 webmail postfix/sendmail: fatal: firstname.lastname@example.org(74): Message file too big
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. reload or restart postfix when you’re done:
service postfix reload
service postfix restart
There’s also mailbox_size_limit directive. You need to change this if SM can not open mailbox sized more than 10 MB.