How does an email travel from one server to another?

You press send, and your email vanishes from your screen. Where does it go? It doesn't teleport directly to the recipient's inbox. Instead, it takes a journey through multiple servers, each one checking credentials and deciding whether to pass it along.

Here's what actually happens, step by step:

Your email client hands off the message to your outgoing mail server. This is called an SMTP server (SMTP stands for Simple Mail Transfer Protocol, though there's nothing simple about what happens next). If you're using Gmail, Outlook, or an ESP like Mailchimp, their servers handle this part for you.

The sending server looks up where to deliver the message. It queries DNS (the internet's address book) for something called an MX record. That's a Mail Exchange record, which tells the sending server which mail server accepts mail for the recipient's domain. If you're emailing captain@deepcurrent.io, the sending server asks DNS, "Who handles mail for deepcurrent.io?" DNS responds with an IP address.

The two servers connect and negotiate. Your sending server reaches out to the recipient's server over TCP (the internet's handshake protocol) and says, "I have a message for captain@deepcurrent.io." The receiving server might accept it immediately, or it might ask for proof that you're allowed to send from your domain. That's where SPF, DKIM, and DMARC come in.

The receiving server makes a decision. It can accept the message (it goes into the recipient's mailbox), defer it ("try again later"), or reject it outright. This decision depends on spam filters, authentication checks, sender reputation, and the receiving server's own policies. Every step of this conversation gets logged, which is why you can track delivery and troubleshoot when things go wrong.

The message sits on the receiving server until the recipient retrieves it. Once accepted, the email doesn't move again until the recipient opens their email client. At that point, protocols like IMAP or POP3 pull the message from the server to their device. SMTP pushes mail from server to server. IMAP and POP3 pull it from server to client.

The whole process happens in seconds (usually), but it's not a straight line. Emails often pass through multiple intermediate servers (relays) before reaching the final destination. Each relay repeats the same authentication and filtering checks. That's why a message can succeed at one hop and fail at another.

But if you want to see this process in action, check the email headers of any message you receive. Every server that touched the message leaves a timestamp and a note. It's the email's travel log.

Want to understand what happens after the recipient retrieves the message? Read about SMTP, POP, and IMAP next.