Has WP 5.7.2 possibly changed the email process, specifically, does anyone know where to find a list of PHPMAILER_EXCEPTION_CODES?

Been using a Form plugin for several years (forms haven’t changed for some time) and up until recently these forms have been fully functional … specifically … they are supposed to send both a submission email and a confirmation email after each form is submitted … for some unknown reason, the confirmation emails have stopped working

Wondering if one of, or combination of, 4 recent system updates may be involved;

  • WP 5.7.2
  • Centos 7.9
  • PHP 7.4
  • VFB Pro 3.4.4

So far I have;

  • regressed PHP to 7.2 (no change)
  • did a file comparison of the Plugin Update which indicates all changes appear to be unrelated to email
  • added some "log to file" debug statements to wp/wp-includes/pluggable.php function wp_mail() which indicate the plugin’s confirmation Email Call is throwing an Exception Error at line 535 $ mail_error_data[‘phpmailer_exception_code’] = 1

Any thoughts as to what does this exception code might mean?

The confirmation email does have; To, Subject, From, and Body/Headers

The Plugin’s Submission Email works properly

Could the content of an email possibly be throwing this process?

function getcode() doesn’t reveal any meaning for this exception code

Frustratingly hard to fix something without knowing what is actually causing the issue

Are there any other methods of debugging these kinds of issues without brute force debug statements?