I've noticed a lot of emails being sent out in my campaigns are getting stuck in pending. I've created stop dates on email campaigns, but it appears that some emails are being sent out after the stop date if they were stuck in pending. Does Marketo not pull out the pending emails if it will be actually sent out past the stop date? There are some emails that are time sensitive and not relevant after a particiular date.
Once you've hit send, the messages go out, and you can't really recall them. Pending is a status which I believe refers to messages that are still attempting to deliver. They've already been sent by Marketo, but they haven't yet been received by the recipient's mailbox (but they haven't been marked as bounced yet). There's some good dialogue about this here: 'Pending' Emails?
The "stop date" function works to limit future runs of the campaign itself. So, for example, if you had a recurring campaign going out each week, setting a stop date would prevent that from recurring after a certain date. It can't, however, stop a send or recall messages after they've been initiated though.
Usually emails in a "pending" status are only there for a few hours at most. If you're seeing a longer lag time than that, you might want to submit a ticket to support, just to double check everything on the deliverability front is functioning.