Emails sent/received at the same time - how to ensure?

There is generally always going to be a little bit of latency, and this can vary based on the size of the list you are sending to, the structure of your instance, what else is going on etc. 

1 hour 16 minutes seems a bit long though. Did you have a very large send or were there a high volume of other activities going on? 

Tip: when you initiate the send you can look at the campaign queue to get an insight into what activtities are happening in your instance and how Marketo is prioritizing them. 

@David L This requirement is impossible. Even if you could send every single email at the same time, there is no guarantee that the recipients' servers will all accept them at that time.

But you can't send every single email literally at once because that technical model would decimate Marketo's (and any mass emailer's) business model.  The service must be allowed to amortize resource utilization to a reasonable degree. If you run across a multi-tenant service that guarantees that every recipient in a massive list will be attempted at once, regardless of the list size and regardless of resources in use by other tenants... that service is a scam.

If you are running a contest, start the contest a couple hours after the send begins.  You can have people RSVP (via click) as they get emails and start an "exciting" countdown timer for them, but don't expect to start the contest in 5 minutes.  That's not how email works.

@David L, Based on my conversations with Marketo's support, 100 emails per minute is a good rule of thumb to help set your expectations.