"Pending" generally means that Marketo is attempting to deliver a message. Marketo will try to send a message, but can receive a temporary bounce (essentially "Thanks, try again later"). When that happens, the mail status is set to pending while we retry. We will try to send the message several times over the next 24 hours (36 to AOL) before we accept that the message will not be delivered and change the status to "bounced".
Occasionally, other Marketo server issues can cause emails to be stuck in Pending. If all the emails in Pending are from consumer email addresses, such as Yahoo, Hotmail, or QQ.com, then we are probably just waiting for the receiving server to accept the message. However, if the pending emails are to a variety of different business domains, please contact Marketo support and we can investigate.