Go to Server Configuration and select Advanced tab. Change the value for Retry Interval and Retry Count. Product of these two values represent minutes any email will sit in the OutboundQueue. For example, if you have 10 for Retry Interval and 144 for Retry Count, that mean emails will by retried every 10 minutes for 1 day (10 * 144 = 1440 minutes or 24 hours).