Allow user to send email without authentication

Create a plain text file: /etc/postfix/accepted_unauth_senders:

fax-machine-12@mydomain.tld OK

Use postmap to create hash db file:

# postmap hash:/etc/postfix/accepted_unauth_senders

Modify Postfix to use this text file: /etc/postfix/main.cf

smtpd_sender_restrictions = 
    check_sender_access hash:/etc/postfix/accepted_unauth_senders,
    [...OTHER RESTRICTIONS HERE...]

Restart/reload postfix to make it work:

# /etc/init.d/postfix restart

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