Allow SMTP AUTH only from mynetworks

You'll likely want to use smtpd_sasl_exceptions_networks.

Thanks @84104. Your answer led me to find this topic which solved my problem.

smtpd_sasl_exceptions_networks does 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_networks is obsoleted by smtpd_discard_ehlo_keywords and smtpd_discard_ehlo_keyword_address_maps. The latter two prevent Postfix from accepting AUTH commands.

smtpd_sasl_exceptions_networks should probably be deprecated and eventually removed from documentation.