Connect to private/dovecot-auth failed

About
Connect to private/dovecot-auth failed is an error message which appears in. It is generated by Postfix, but is actually caused by the failure of a Dovecot component which is supposed to provide a pipe for Postfix to talk to. The pipe is usually located at /postfix.

A more complete and generalized version of this message is:
 * servername nss-mysql[ number ]: warning: SASL: Connect to pipe failed: No such file or directory

Where:
 * servername is the network name of the machine generating the error
 * number is an integer, significance unknown (it seems to be a standard thing in mail.log files)
 * pipe is the filespec of a pipe for communication between Postfix and Dovecot, typically "private/dovecot-auth"
 * (is "pipe" correct, or do I mean "socket"? not sure.)

In one particular scenario, it is preceded by an SMTP connection attempt from a remote server, and followed by a fatal: no SASL authentication mechanisms error message.

fixes
One fix involved the following:
 * ls /var/spool/postfix/private
 * look for pipe with "auth" in the name
 * modify Dovecot's file so that the auth default section includes at least the "client" section shown here, but with the "path" parameter pointing at your actual auth pipe.

If that doesn't work, go to the Postfix and Dovecot web sites and read up on SASL, especially this section.