SMTP stands for Simple Mail Transfer Protocol, the procedure behind the email flow on the internet.

• You send an email with your webmail or mail client from your address (e.g. [email protected]) to a given recipient (e.g. [email protected]). In jargon, the webmail or client is called Message User Agent, or MUA.

• The message is sent normally via port 25 to an SMTP server (named for instance mail.website.com) which is given to your client when you set it up and acts as a Message Transfer Agent or MTA. Client and server start a brief “conversation” where the latter checks all the data concerning the message’s transmission (sender, recipient, domains, etc.). Note that SMTP language defines only the message’s transmission, and doesn’t deal with its body content.

• Then, if the domain where your recipient has his account is directly connected to the server, the email is immediately delivered. If it’s not the case, the SMTP hands it to another incoming server closer to the recipient (in jargon these passages are called relays). In our example, the Website server connects with the Domain server, which (if everything has gone right) receives the email and stores it.

• What if the recipient’s server is down or busy? The SMTP host simply drops the message to a backup server: if none of them is available, the email is queued and the delivery is retried periodically. After a determined period, however, the message is returned as undelivered.

• If there are no issues, however, the final segment is controlled by POP, another protocol that picks up the email from the receiving server and puts it into the recipient’s inbox.