From LedHed's Wiki
To enable Postfix (v 2.3+) to use Dovecot's SASL add the following to main.cf
smtpd_sasl_type = dovecot smtpd_sasl_path = /var/spool/postfix/private/auth smtpd_sasl_auth_enable = yes
There are many other SASL related settings that also need to be set for proper SASL configuration. See Postfix SASL