We sent out a campaign just over 24 hours ago to a database of 4,800 and still find 14% of the emails as "pending". We're curious to know why they are pending, and how much longer they could possibly remain pending?
It sometimes takes the Marketo servers 36 hours or more to process pending emails. You might track those that are pending so that you watch them in other campaigns. It's possible they may be bad adreses too.
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!