Lo más importante es revisar las siguientes líneas de configuración

# SASL SUPPORT FOR CLIENTS

# The following options set parameters needed by Postfix to enable 

# Cyrus-SASL support for authentication of mail clients. 

# 

smtpd_sasl_auth_enable = yes

smtpd_sasl_security_options = noanonymous

smtpd_sasl_local_domain = $myhostname

broken_sasl_auth_clients = yes

... 

smtpd_recipient_restrictions = 

   permit_sasl_authenticated, 

   permit_mynetworks, 

   check_relay_domains