Thank you for the update, Maxwell. Emails have been sending, albeit very slowly. I’m not sure if this has to do with not having updated class.phplistmailer.php and class.smtp.php.
As notes in this tutorial, class.phplistmailer.php is located in the admin/PHPMailer/ directory, as is class.smtp.php (which only seemed to have a variable setting for the port.
I’m guessing I will need to either keep these files separate when upgrading, or repeat the process upon upgrade.