How does an ESP send emails?

When you hit 'send' in your ESP, a sophisticated pipeline activates. First, the ESP validates your recipient list against suppression lists (bounces, unsubscribes, complaints). Then it renders personalized versions of your template, generates unique tracking links and pixels, and queues messages for delivery through its MTA infrastructure.

The MTA establishes SMTP connections with recipient mail servers, performing DNS lookups to find each domain's MX records. It negotiates TLS encryption, presents authentication credentials (SPF, DKIM), and transmits the message. Receiving servers respond with acceptance, deferral, or rejection codes. The ESP logs everything and processes bounces, opens, and clicks as they occur.

Behind every 'send' button lies an orchestra of protocols. ESPs manage connection throttling (respecting mailbox provider rate limits), retry queues for deferred messages, IP rotation strategies, and real-time reputation monitoring. They're sending millions of messages per hour while making the complexity invisible to you. Understanding this pipeline helps you appreciate why deliverability issues can be complex—there are dozens of potential failure points between your click and your subscriber's inbox.