Пример настройки работы Postfix через релэй с авторизацией наподобие krasmail.ru:

/etc/postfix/main.cf

   relayhost = [krasmail.ru]

   # auth_krasmail
   smtp_sasl_auth_enable = yes
   smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd
   smtp_sasl_security_options = noanonymous
   smtp_sasl_type = cyrus
   smtp_sasl_mechanism_filter = login
   smtpd_sasl_path = smtpd

/etc/postfix/sasl_passwd

   [krasmail.ru]  user:password

и потом 

   postmap sasl_passwd
   postfix reload