On some server installations, mail server login attempts cause HELO/EHLO error on PHP 8+ like

rejected EHLO from web.site.ru [192.168.0.1] I=[192.168.0.1]:465: syntactically invalid argument(s): (no argument given)

rejected HELO from web.site.com [192.168.0.1] I=[192.168.0.1]:465: syntactically invalid argument(s): (no argument given)

The problem is unexpected output of gethostbyaddr() for some reasons (f.e. PHP bug like https://github.com/php/php-src/issues/7748 and/or changes of gethostbyaddr() PHP8 src logic etc) so that the result is a binary or empty string.

In this case empty result shouldn't be used as HELO/EHLO argument.