If you encounter the following error after updating from the Swiftmailer module to the Symfony module:

Connection to “process/usr/sbin/sendmail -bs” has been closed unexpectedly. 

This issue has also been reported on the drupal.org community website: https://www.drupal.org/project/symfony_mailer/issues/3343418

RESOLUTION
Follow the instructions in this article: Sendmail Settings for Drupal Symfony Mailer and Drupal Symfony Mailer Lite.