Hi Jenn, We were having the same issue. My colleague submitted a service desk ticket and this was the response she received from Marketo:
"Our Engineering and Operations teams have identified an issue with emails not being successfully delivered.
Campaigns and programs will show that the emails have been sent but none of them delivered. This is affecting sample email sends as well.
We apologize for the trouble and we are treating this problem as a Priority 1 Emergency. I’ll provide you with updates as soon as new information is available.
We appreciate your patience while we resolve this and our teams are actively working to restore services as soon as possible. "