Make sure smtpd_recipient_restrictions has permit_mynetworks, something like this:
smtpd_recipient_restrictions = permit_mynetworks,<more>
This is the default, but if you have -o smtpd_recipient_restrictions=something on an smtpd line in master.cf then it will override main.cf setting.
A sample configuration that worked for me is as below-
smtp inet n - n - - smtpd -o smtpd_sasl_auth_enable=yes -o smtpd_reject_unlisted_sender=yes -o smtpd_recipient_restrictions=permit_sasl_authenticated,permit_mynetworks,reject # -o smtpd_recipient_restrictions=permit_sasl_authenticated,reject -o broken_sasl_auth_clients=yes
