How Does SMTP Relay Work?
Here is the SMTP delivery process in a nutshell. Once a message is composed by a sender, it needs to be submitted to outgoing email relay to begin its transfer across the Internet. The “Send” button is pressed, and the user-level mail client uses the Simple Mail Transfer Protocol to “talk” to the SMTP server, usually on the default TCP port 25. The outgoing email server receives the message, saves it locally and queues it to be relayed along with other emails received. Since a direct connection between the users is not always possible, the path may be broken up into hops. The SMTP server looks up the destination domain`s Mail Exchange (MX) record in the Domain Name System (DNS), and relays the message to a server listed in the record, again via port 25 and SMTP. Each successive SMTP session delivers the electronic message one hop further along the path toward the recipient. The process repeats itself at each hop until the message reaches the destination post office, assuming the delive
