fatal: no SASL authentication mechanisms && warning: xsasl_cyrus_server_get_mechanism_list: no applicable SASL mechanisms

If you see your Postfix reporting:

Jun 4 10:45:35 host postfix/smtpd[13609]: warning: xsasl_cyrus_server_get_mechanism_list: no applicable SASL mechanisms
Jun 4 10:45:35 host postfix/smtpd[13609]: fatal: no SASL authentication mechanisms

and you already did yum install cyrus-sasl-plain and checked and didn't find anything suspicious in the configuration settings, then most probably you have duplicated entries in /etc/sasl2/smtpd.conf. Making sure you have unique entries in that file won't necessarily rid of the error messages, however at least it eliminates one possible reason.