How does an ESP send emails?
Still have a question, spotted an error, or have a better explanation or a source we should cite?
You hit send. The ESP takes it from there. But what actually happens in between? More than you might expect.
The first thing your ESP does is prep your list. It checks every address against its suppression list, which holds anyone who previously bounced, unsubscribed, or marked your emails as spam. Those addresses are quietly removed before a single message goes out.
Next, it builds each individual email. Your template gets personalized with each recipient's data, tracking pixels get embedded, and links get wrapped so clicks can be recorded. What comes out is a formatted MIME message (the standard structure all email clients understand).
Then the actual sending begins. Your ESP hands each message to an SMTP server, which is the protocol that moves email from one server to another. That server performs a DNS lookup to find the receiving domain's MX record, which is basically the address of the inbox it needs to deliver to.
Once it finds the right destination, it opens a connection, negotiates encryption via TLS, and presents your SPF and DKIM credentials so the receiving server can verify the email is legitimate. The receiving server then responds with one of three outcomes. It accepts the message, defers it (try again later), or rejects it outright. Your ESP logs every one of those outcomes in real time. Bounces get processed, click and open data flows back in, and deferred messages go into a retry queue. It also manages connection throttling so it doesn't overwhelm any one provider's servers, which would hurt your reputation fast.
So the whole thing happens at enormous scale, often millions of messages per hour, without you seeing any of it. That's the point. But knowing these steps helps explain why deliverability problems can be tricky to pin down. There are a lot of places between your click and someone's inbox where something can go wrong.
If you want to see how your own setup holds up, our free Email Header Analyzer can show you exactly what happened at each step of a delivery. Or if something's already broken, our SOS hotline is free to use.
Contributors
Who worked on this answer
Every name links to their profile. Every company links to their site. Real people, real accountability.
