Error: Postfix/Dovecot - warning: SASL: Connect to private/auth failed: No such file or directory

You have the following error messages in your mail log:

Aug 23 15:55:01 server1 postfix/smtpd[15194]: warning: SASL: Connect to private/auth failed: No such file or directory 
 Aug 23 15:55:01 server1 postfix/smtpd[15194]: fatal: no SASL authentication mechanisms 
 Aug 23 15:55:02 server1 postfix/master[3979]: warning: process /usr/libexec/postfix/smtpd pid 15194 exit status 1 
 Aug 23 15:55:02 server1 postfix/master[3979]: warning: /usr/libexec/postfix/smtpd: bad command startup -- throttling

Solution

Open your dovecot.conf (/etc/dovecot.conf or /etc/dovecot/dovecot.conf, depending on your distribution) and make sure you have the following lines in it:

[...]
client {
       path = /var/spool/postfix/private/auth
       mode = 0660
       user = postfix
       group = postfix
}
[...]

Restart Dovecot.

Then check if /var/spool/postfix/private/auth got created:

ls -l /var/spool/postfix/private/auth

Also make sure you have

[...]
queue_directory = /var/spool/postfix
[...]

in /etc/postfix/main.cf. If not, add it and restart Postfix.