We had a large Cyber Monday send this morning to about ~4.2 million. It send to a static list and we did head start overnight. Started at 6 am and its still not done delivering. Is this normal for this volume?
I would not be surprised, that is a huge volume to go through. Also important thing to note, Marketo may have sent the emails, but Marketo does not flag it as delivered until the recipient servers accept the emails or considers it as a soft/hard bounce.
However, I would definitely log a Marketo support ticket and ask them to check into your instance to see if there is any abnormality.