If you see following error in your syslog:
warning: SASL authentication failure: cannot connect to saslauthd server: No such file or directory
Then, the solution is as is in https://nfolamp.wordpress.com/2013/02/04/fixing-postfix-and-saslauthd-ca...

Here are required commands:

ln -s /var/spool/postfix/var/run/saslauthd /var/run
chown root:sasl /var/spool/postfix/var/run/saslauthd
usermod -a -G sasl postfix
/etc/init.d/postfix restart
/etc/init.d/saslauthd restart
/etc/init.d/courier-authdaemon restart