Issue
You may encounter this error message in the ajax_functions.php file or similar when the PHP function checkdnsrr() is called but not found:

Call to undefined function checkdnsrr()

This error occurs because the PHP DNS functions extension (which includes checkdnsrr() ) is  missing or disabled on your server.

Why This Matters

The checkdnsrr() function is commonly used to verify email addresses by checking DNS records. It helps prevent the registration of invalid or suspicious email addresses.

Recommended Solution

Enable the PHP DNS extension on your server.

For Linux servers, this usually means installing or enabling the php-dns or php-common package.

On Windows, ensure your PHP build includes this extension.

Contact your hosting provider if you do not have direct control over PHP modules.

Reference:
https://www.php.net/manual/en/function.checkdnsrr.php