With PHP 7.4 and lower this kind of Line endings works fine:
\phpBB\includes\functions_messenger.php

 $encode_eol = ($config['smtp_delivery']) ? "\r\n" : PHP_EOL;

Which leads to "\n" als $eol.

From PHP 8.0 this needs to "\r\n" for e-mails or the headers will leads to problems with special characters like äöü or other UTF-8 characters.

Suggested solution

$encode_eol = $config['smtp_delivery'] || PHP_VERSION_ID >= 80000 ? "\r\n" : PHP_EOL;