chrooted: localhost not reachable & php mail

Summary

php mail() is not working on chrooted websites (php-fpm), still after changing SMTP = localhost to SMTP = 127.0.0.1 in php.ini-file. localhost is not reachable via chrooted, but that's not the problem with php mail() I think.

Steps to reproduce

Enable chroot-option for website
Run example php sendmail script on website
Check mail logs

Correct behaviour

php mail() should also work in chrooted.

Environment

Server Debian 11 latest ISPConfig version: 3.2.7p1

Proposed fix

Open /etc/php/VERSION/fpm/php.ini & change:

SMTP = localhost

to:

SMTP = 127.0.0.1

After that I have no more clue (fix for first step) :D