Obrigado @ 84104. Sua resposta me levou a encontrar este tópico que resolveu o meu problema.
smtpd_sasl_exceptions_networksdoes not announce AUTH support but still allows AUTH from any IP.This feature was donated to prevent certain network clients from messing up when the server announces AUTH support but the client has no login information. With
smtpd_sasl_exceptions_networks, those clients would not try to authenticate and all was well.
smtpd_sasl_exceptions_networksis obsoleted bysmtpd_discard_ehlo_keywordsandsmtpd_discard_ehlo_keyword_address_maps. The latter two prevent Postfix from accepting AUTH commands.
smtpd_sasl_exceptions_networksshould probably be deprecated and eventually removed from documentation.