Unable to send mail: SMTP Issues

WP Mail SMTP 1.4.2, WordPress 5.1.1, Ubuntu 18.04 LTS on AWS EC2. Mail server is Postfix SMTP / Dovecot IMAP on another Ubuntu 18.04 LTS on EC2 (separate instance.) Mail is working fine otherwise. I’m receiving mail from clients and vendors fine. I’m able to send mail from all my various mail clients and plug-in’s fine.

SMTP port is 143 with STARTTLS (Or straight TLS.)

The only thing I see in my mail logs when I test a connection from WP Mail SMTP is a connection from:
a27-154.smtp-out.us-west-2.amazonses.com[54.240.27.154]

This is not related to any other mail traffic that I can see, and it only hits the logs when I try to send a Test Mail from WP Mail SMTP.

This *was* working fine. I did not change anything on my mail server, and like I said everything else relating to email is working fine.

I did install WordFence Security for WordPress, but I disabled it to make sure it was not causing this issue. I don’t know what other logs to look at.

In my Apache2 error.log:

[Thu Apr 11 15:43:13.806003 2019] [php7:error] [pid 1671] [client 72.209.60.218:48575] PHP Fatal error: Uncaught Error: Class ‘WPMailSMTP\\Debug’ not found in /var/www/html/wp-content/plugins/wp-mail-smtp/src/Providers/Loader.php:182\nStack trace:\n#0 /var/www/html/wp-content/plugins/wp-mail-smtp/src/Providers/Loader.php(133): WPMailSMTP\\Providers\\Loader->get_entity(‘smtp’, ‘Mailer’)\n#1 /var/www/html/wp-content/plugins/wp-mail-smtp/src/Admin/Pages/Test.php(464): WPMailSMTP\\Providers\\Loader->get_mailer(‘smtp’, Object(WPMailSMTP\\MailCatcher))\n#2 /var/www/html/wp-content/plugins/wp-mail-smtp/src/Admin/Pages/Test.php(189): WPMailSMTP\\Admin\\Pages\\Test->get_debug_messages(Object(WPMailSMTP\\MailCatcher), ‘2019-04-11 15:4…’)\n#3 /var/www/html/wp-content/plugins/wp-mail-smtp/src/Admin/Area.php(403): WPMailSMTP\\Admin\\Pages\\Test->process_post(Array)\n#4 /var/www/html/wp-includes/class-wp-hook.php(286): WPMailSMTP\\Admin\\Area->process_actions(”)\n#5 /var/www/html/wp-includes/class-wp-hook.php(310): WP_Hook->apply_filters(NULL, Array)\n#6 /var/www/html/wp-includes/plugin.php(465): WP_Hook->do_action(Array)\n#7 /var/www/html/wp-ad in /var/www/html/wp-content/plugins/wp-mail-smtp/src/Providers/Loader.php on line 182, referer: https://www.rogerprata.com/wp-admin/options-general.php?page=wp-mail-smtp&tab=test

I have no idea what the hell any of that means, or if it has anything to do with this issue.

I would appreciate any assistance. Thank you.