Marketo should create a feature like most ESP's where you can throttle volumes to deploy at specified rate, i.e 5k per hour.
This helps from a deliverability/inbox placement perspective as some SPAM filters look at whether a campaign is a huge volume mail blast to all, instead of a "steady as you go" strategy when it comes to managing large volumes.
For some Marketo customers, I'm sure managing the traffic reaching call centres directly from an email campaign would come in handy!
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