Excerpt /etc/postfix/main.cf
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transport_maps = hash:/etc/postfix/transport
smtp_sasl_auth_enable = yes
smtp_sasl_security_options = noanonymous
smtp_sasl_password_maps = hash:/etc/postfix/sasl_password
sender_canonical_maps = hash:/etc/postfix/sender_canonical
smtp_tls_security_level = encrypt
Excerpt /etc/postfix/transport
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<mailaddress>@gmx.net smtp:mail.gmx.net:25
Excerpt /etc/postfix/sender_canonical
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root@proxmox sberg1@gmx.net
Excerpt /etc/postfix/sasl_password
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mail.gmx.net <mailaddress>@gmx.net:<password>
Execute postmap commands to create the hash db file from above transport / sender_canonical / sasl_password
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postmap transport
postmap sender_canonical
postmap sasl_password
Restart Postfix
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systemctl restart postfix
Testing
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mail -s test <mailaddress>@gmx.net
HINT: CC just hit <Enter> – then write the body followed by <Enter> and <.> (dot character) and again <Enter>- then quit the mail command by pressing <CTRL>-<D>