If you get an error like:
Apr 15 12:41:03 webb4 postfix/sendmail[11234]: fatal: web1_user(12967): queue file write error
in your mail log, then the value for message_size_limit in postfix main.cf is too low.
Solve Postfix queue file write error
Edit postfix main.cf:
nano /etc/postfix/main.cf
and set the message size limit to a higher value. E.g.
message_size_limit = 900000000
and restart postfix:
service postfix restart
If there is no message_size_limit set in main.cf, just add the line.
