This is a list of problems related to server configuration problems when running OG Mailinglist.
Drupal files belong to nonexistent user
postfix/sendmail: fatal: no login name found for user ID 6226
For some reason, the drupal install. directory and all children were owned by user 6226 from group 6226. The problem went away once I changed the ownership.
chown -R www-data:www-data /var/www/drupal
Thank you, sano, for this tip.