Problem with process_mailing
OK, the answer turns out to be a matter of capitalization. I'm using the cv addon (https://github.com/civicrm/cv) to send the command "job.execute" from cron. My cron line looks like:

cv api job.execute --user=username --cwd=/var/www/html/sites/default


However, my username on the server is not capitalized while my Username in Drupal IS capitalized. When I changed the cron to "--user=Username" all is well. It is interesting that the cron WAS executing and that the commands were being fired, thus the entries in the CiviCRM logs. However, since Drupal was seeing "username" as an unauthenticated user without the authority to send email, even though the process_mailing command was working, SMTP was failing.