It is controlled by a setting under System Setting called "Retry Mode" This setting allows you to mark an email address as retry later after a certain number of blocked/timed out hits on that domain. The default is five blocked/timed out attempts before the email is added to the retry queue for that campaign. You can enter a value of zero to disable the retry mode system but that is not recommended.
The retry mode should help improve reputation since the isp's will not be sent excessive emails and help improve the warmup period. The retry mode is automatic and by default try once every 24 hours five times. This process starts when the campaign has been completed.
You can also send them manually if the campaign is completed on the view campaign there will be a send retries link next to retry queue total. Clicking that link will take you to a retry schedule page to let you choose a date/time to resend the retries. After scheduling the retries you will be taken to the Retry Queue Status page. On the Retry Queue Status page, you can monitor the progress of the retries. If you need to access the retry queue page again there is a button from the campaign status page that will take you there.
Note also the retry queue does use up broadcasters as well to send the retries.
Note: With the retry mode system campaign speeds will increase since emails are sent to retry pool on failure.
Note: Retry queue sending does not use any sending credits!!