Does the SimpleTest method drupalGetMails() prevent actual emails from being sent?
That is only the getter.

// Use the test mail class instead of the default mail handler class.
variable_set('mail_system', array('default-system' =&gt; 'TestingMailSystem'));


in DrupalWebTestCase::setUp stops mails being sent. TestingMailSystem does nothing else but collect emails in the drupal_test_email_collector variable.