Sender address rejected: not owned by user firstname.lastname@example.org
This error is caused by Postfix setting:
smtpd_recipient_restrictions = ... reject_sender_login_mismatch, ...
It will reject the request when $smtpd_sender_login_maps specifies an owner for the MAIL FROM address, but the client is not (SASL) logged in as that MAIL FROM address owner; or when the client is (SASL) logged in, but the client login name doesn't own the MAIL FROM address according to $smtpd_sender_login_maps.
Remove reject_sender_login_mismatch in Postfix config file (/etc/postfix/main.cf) and restart Postfix can fix it.