I was working setting up a new Managed VPS for a client today. The client wanted to host their emails on the server so I was setting up Postfix mailserver. The server was Ubuntu 16.04 with Virtualmin and Nginx.

I installed Roundcube for webmail. But when I try to send a test mail I got Postfix SMTP Error (535): Authentication failed.

After researching for a bit, I've found out that I was missing the missing the symlink from /var/run/saslauthd to /var/spool/postfix/var/run/saslauthd. So I created it with following command.

ln -s /var/run/saslauthd /var/spool/postfix/var/run/saslauthd

That fixed the Postfix SMTP Error (535): Authentication failed. Remember to restart Postfix and saslauthd.