Customized entries in
/etc/postfix/main.cf got reverted after the command
plesk repair mail was launched, Plesk microupdates, or after applying settings in Tools & Settings > Mail Server Settings.
Plesk has a whitelist of values for some of the entries in
/etc/postfix/main.cf and automatically removes unlisted values.
A vote can be left on Plesk UserVoice for allowing
/etc/postfix/main.cf protected entries customization: Allow customization of /etc/postfix/main.cf.