From iRedMail

Sender address rejected: not owned by user user@domain.ltd

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 and restart Postfix can fix it.

Reference: http://www.postfix.org/postconf.5.html#reject_sender_login_mismatch