Email Throttling

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... dump 300,000 emails into circulation all at once? Such a vast quantity of emails deployed in such a small amount of time raises a red flag for many ESPs, who may spam a lot of the emails and maybe even blacklist the sender.

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“Into circulation” is a bit vague, since 300,000 connections to the same MX is very different from 300 connections to 1000 different MXs.

You also can’t compare a (largely) trusted mass sender like Marketo to, say, spinning up your own mailserver on a new IP and queueing 300,000 messages. Marketo users benefit from economies of scale. Systems that regularly send 100s of millions of messages per day with relatively few spam reports are allowed to continue that trend: that‘s part of the algorithm used by major antispam providers such as MessageLabs, Barracuda, et al.

All that said, you’re right that there are mailservers that set low enough limits that you can’t reasonably send to them using a generic Marketo send. Those limits, though, are more likely to be per day than per hour, so hourly throttling isn’t typically the answer. Marketo will retry messages for up to 24 hours in response to non-permanent SMTP errors, such as “server too busy” or “max connections” exceeded, using a standard exponential backoff algorithm (waiting longer and longer between retries to give the recipient time to breathe). Inspecting your bounces will tell you whether this logic is still stopping people from getting email. If you need to space sends out over days, use Random Sample, or a more robust webhook-based sorting method, to “curate” what you send each day.