MailChimp currently sends about 600 million emails every day, and all of those users expect their campaigns to be delivered immediately. Conveniently, that’s exactly what we want, too. So how do we make it happen? We approach sending those 600 million emails like this golden retriever approaches a big plate of spaghetti: with no fear, just wolfing it all down super fast in big chunks.
Our sending infrastructure is designed to turn large campaigns into smaller “payloads” to get them out the door much faster. When you click Send in the Campaign Builder, you’re actually telling MailChimp to start biting off parts of your campaign. As each payload is created, it’s immediately routed to our Mail Transfer Agents (MTAs) and queued for delivery. We organize this based on subscriber member ratings, so the most engaged subscribers in your campaign are first in line.
For larger campaigns, like the one in the following graph, payloads usually only include about 10,000 recipients each. Here, since the MTA isn’t waiting around for one giant payload of 470,000 to be built and queued at once, deliveries can start immediately.
Once all of those small payloads are queued, we mark your campaign as “sent” and begin monitoring responses from each of your recipients’ servers for delivery confirmations or bounces. As you can see below, campaigns with less than 100,000 recipients can deliver to the majority of those subscribers in less than 15 minutes.
When we built the system to create and send in small payloads, we had big senders in mind. But, as it turns out, everyone benefits—smaller senders will never be stuck waiting in line behind a bigger user.
All this payload building and routing helps make MailChimp the spaghetti-gobbling dog champion of the world! Um, I mean, one of the fastest bulk senders in the business.
[Originally posted here, but due to MailChimp’s rebranding in October 2018, the MailChimp blog was removed.]