Email bug when send more than 4 or 5 messages

In a relatively recent install of Drupal 7.19 (don't chide me for that), I've set the Mail System class to "SMTPMailSystem". I have a view, using VBO, of selected users. When I attempt to send a mass email to 12 of them, (1) I get an indecipherable error dump after successfully sending 4 emails, and (2) the Mail System class resets itself to DefaultMailSystem. The Default Mail System delivers to localhost, not to my smtp server, and we don't run a mail server on localhost. Although I need to update core drupal, the Mail System module is current and so are VBO and Views Send. If attached the debug display. Any help or direction would be appreciated.