What are the main steps in email delivery?

Email delivery follows a predictable voyage from sender to recipient. First the client hands the message to an outbound MTA, which checks authentication and prepares the envelope. The MTA performs a DNS lookup to find the destination MX record. Once it discovers the correct receiving server, it opens an SMTP connection to hand off the message. The receiving MTA accepts the message, applies filtering, and either delivers it to the mailbox or routes it to spam or quarantine.

Behind the scenes, every hop leaves a mark in the Received headers which trace the journey like stamps in a sailor’s passport.