If the Postfix configuration file is missing default directives and its last line does not end with a newline character, using postconf to modify the configuration file or updating/downgrading Postfix can yield a non-working config.

This happens because whenever postconf wants to add a line, it gets appended to the end of the last line, instead of being on a line of its own.

This can be reproduced by removing the newline at the last line of the file and then running a postconf command to set a directive, e.g.:

# postconf virtual_minimum_uid=100  

Alternatively, removing any of the default directives, as well as the newline at the last line of the file, will cause this behaviour to occur whenever Postfix is updated/downgraded via yum/dnf. This happens because during the post-install phase, "postfix set-permissions upgrade-configuration" is called, which in turn uses postconf internally.