@MikeVL Did the problem start happening after you upgraded to phplist 3.5.8? If so then you can try reverting to use phpmailer 5 instead of phpmailer 6, which became the default in 3.5.8. Add this line to the config.php file
define('USE_PHPMAILER6', false);
If you are sending to a range of domain, yahoo, gmail, hotmail etc, then it doesn’t seem likely that they would all reject all emails for the same reason. Maybe it is a problem with the smtp server through which phplist is configured to send.
Can you copy a complete bounce email here, use the toolbar code button </> so that it is displayed as plain text.