Marketo is in the process of updating our mail infrastructure. One of the improvements, Marketo will now retry mail for 24 hours, up from 4 hours, if it is unable to deliver or record a bounce on the first pass. This will mean you will have more mail in Pending status for longer periods of time, but it will also equal higher rates of delivered email.
If you do a mailing and you have a large number of mails stuck in pending for longer than 24 hours, please contact Support to have us investigate!