> But no matter what, any action in smtpd.conf is a command that is going > to get executed and a process has to have a owner, so there is going to > be a system user involved.
Thank you for the explanations, this is clearer. What about RFC 1891? Is there an option to disable use of additional parameters such as ORCPT  to ensure compatibility with smtp tools that does not support this standard?  https://tools.ietf.org/html/rfc1891#section-5.2