Connect to private/auth failed: No such file or directory

Check

/etc/dovecot/conf.d/10-master.conf

for line

unix_listener private/auth

change it to

unix_listener /var/spool/postfix/private/auth

Restart dovecot and try again. If still doesn't work, post output of dovecot -n and postfix -n

After spending some hours trying to figure out this, I finally fixed my problem, hope this helps to others...

In the postfix/main.cf file you have something like:

smtpd_sasl_path = private/auth

and in the dovecot/conf.d/10-master.conf you have something like:

service auth { 
    # auth_socket_path points to this userdb socket by default. It's typically 
    # used by dovecot-lda, doveadm, possibly imap process, etc. Its default 
    # permissions make it readable only by root, but you may need to relax these 
    # permissions. Users that have access to this socket are able to get a list 
    # of all usernames and get results of everyone's userdb lookups. 
    unix_listener /var/spool/postfix/private/auth { 
      mode = 0666 
      user = postfix 
      group = postfix 
    } 

  ...

in my case the "private/auth" in the first file was not matching the unix_listener path, as soon as I put this one equal to the first it solved the problem. My guess is that the smtpd_sasl_path has to be the same as the /var/spool/postfix/... otherwise it will fail.

Hope this helps,

Before doing any of re-jigging of your paths, check the dovecot configuration syntax with:

dovecot -n

That command should dump the dovecot configuration file to screen, if it does not then it will show the first error it encountered. Edit the file to fix errors, if any and you should be good to go. If you get a Fatal: Error in configuration file /etc/dovecot/dovecot.conf line 6: Expecting '=' or similar, then simply deleting the spaces before and / or after the equals sign on the denoted line should fix it (I think it is an encoding issue).