In order to protect both OutboundEngine's and your company's email reputation, all new contact lists must undergo a test or optimization phase to increase deliverability and reduce spam abuse.
When new contacts are added to your OutboundEngine account, our system selects a sample of your contact list (at random) to send to in order to optimize deliverability. If the majority of your list is found to be valid, the remainder of the emails will be emailed over the next five days.
Any contacts held back by the test/optimization phase are labeled as "Pending" in the status column on the contacts tab of your Outbound Engine account. Barring any unforeseen issues with the testing/optimization phase, those emails will eventually become "Active" over the span of five days.