Thank you so much for your valuable advice. (Yeah, I really mean it. Thank you so much!!!)
Here's my new config: sudo postconf -n alias_maps = hash:/etc/aliases always_add_missing_headers = yes biff = no config_directory = /etc/postfix disable_vrfy_command = yes home_mailbox = Maildir/ mailbox_size_limit = 0 milter_default_action = accept milter_protocol = 2 mydestination = subexample.example.com, example.com, localhost.example.com, localhost mydomain = subexample.example.com mynetworks = 127.0.0.1/32 [::1]/128 myorigin = subexample.example.com non_smtpd_milters = inet:localhost:8891 smtpd_client_restrictions = reject_rhsbl_client dbl.spamhaus.org, reject_rhsbl_sender dbl.spamhaus.org, reject_rhsbl_helo dbl.spamhaus.org smtpd_milters = inet:localhost:8891 smtpd_recipient_restrictions = permit_mynetworks, reject_unauth_destination, reject_unknown_sender_domain, reject_unknown_reverse_client_hostname, reject_rbl_client zen.spamhaus.org, check_policy_service inet:127.0.0.1:10023 virtual_alias_maps = hash:/etc/postfix/my-valiases
