Postfix_o365_relay_settings.md

#main.cf:
relayhost = [smtp.office365.com]:587

#settings for authentication to o365
smtp_sasl_auth_enable = yes
smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd
smtp_sasl_security_options = noanonymous
smtp_tls_security_level = may

# Relay from this server setting
sender_canonical_maps = regexp:/etc/postfix/sender_canonical

# Relay from other server setting
sender_canonical_classes = envelope_sender, header_sender
smtp_header_checks = regexp:/etc/postfix/header_check

#header_check:
/From:.*/ REPLACE From: email@o365.address

#sasl_passwd:
[smtp.office365.com]:587 email@o365.address:password

#sender_canonical:
/.+/ email@o365.address

#other setup
https://geekdudes.wordpress.com/2018/04/11/configuring-postfix-as-relay-for-office-365/