Version 1.1.18 fails on previous versions of WordPress

For sensible reasons, not all of us upgrade WordPress the moment a new version is released.

So I was surprised to find that, today, my website was no longer able to send SMTP emails when nothing on the server had changed… except I found that SMTP Mailer had been upgraded to 1.1.18 and, when sending email, would fail with the following error:

[16-Apr-2025 18:44:29 UTC] PHP Warning: require_once(/home/xxxx/public_html/wp-includes/class-wp-phpmailer.php): Failed to open stream: No such file or directory in /home/xxxx/public_html/wp-content/plugins/smtp-mailer/main.php on line 586
[16-Apr-2025 18:44:29 UTC] PHP Fatal error: Uncaught Error: Failed opening required ‘/home/xxxx/public_html/wp-includes/class-wp-phpmailer.php’ (include_path=’.:/opt/cpanel/ea-php82/root/usr/share/pear’) in /home/xxxxx/public_html/wp-content/plugins/smtp-mailer/main.php:586

Version 1.1.18 assumes that wp-includes/class-wp-phpmailer.php is available on previous versions of WordPress… it isn’t … and the plugin fails because this file does not exist.