The UCS postfix must authenticate the user if he wants to send an email. There are two SASL possibilities within UCS. On the one hand the stand-alone saslauthd, alternatively the sasl authentication integrated in Dovecot. This is controlled by the UCS variable.
ucr set mail/postfix/dovecot_sasl=no
ucr set mail/postfix/dovecot_sasl=yes
There are no special differences between these two services, only Dovecot wants the domain part as username. So ‘email@example.com’, whereas the saslauthd is satisfied with only the username. Only if you use more than one email domain, there might be some differences in these two services.