Simple Mail Transfer Protocol (SMTP) is a set of rules or standardized protocols for sending and receiving email across networks like the Internet. A computer that runs SMTP is referred to as a mail server, and ideally has a near-constant uptime. The SMTP mail server can both send and receive mail, albeit at the client level we associate SMTP with an outgoing email server, and Post Office Protocol 3 (POP3) with incoming mail.
Email clients require an address for the outgoing email server and the POP3 or incoming server in order to collect and send mail. Internet Service Providers (ISPs) provide these addresses to customers at the time a subscription or contract is enacted, and mail server addresses are also commonly listed on the ISPs’ website. In some cases both outgoing and incoming mail will be handled by a single server, such as mail.[yourisp].com; but often the outgoing email server address resembles smtp.[yourisp].com, and the incoming address, pop3.[yourisp].com.
Authentication is required to access an outgoing email server, consisting of the username and password associated with the customer’s ISP account. This protects the ISP from handling outgoing email generated by non-customers, which could quickly bog down its resources. Additionally, authentication allows the server’s administration to more easily control activity on its outgoing email server to help prevent abuses like spam and fraud.
Once mail is sent to an outgoing email server, the associated SMTP server reads the headers in the email message in order to relay the message to its destination. A dialog begins between itself and the next mail server along the route. The dialog takes the form of a set of requests and responses, which moves the mail forward to its ultimate destination. The mail might travel though several intermediary hosts before reaching the host that serves as the incoming mail server for the recipient. If there is a problem along the way, the mail might be sent back to re-trace its route to the sender, arriving as undeliverable.
A Web-based email service runs its own mail servers which work exactly like an ISP's mail servers. The only difference is that clients log into the website to read, write and send mail, rather than opening a personal email client from the desktop. Webmail such as Gmail® is very popular because it makes mail accessible from any computer with an Internet connection. It also prevents virus-laden mail from being downloaded to your personal hard drive, as the mail arrives (and is read) from the website's server instead.
It is important to note that email should be considered public because it is sent in the clear; a term used for plain text communications sent over a network that can be read by anyone. From the outgoing email server to the many relay hosts, and the final incoming mail server, email is less private than a postcard send through standard mail, and more-so with Web-based services that rely on marketing. Only encrypted email is considered private, as it is put into unreadable cipher before being sent and is unencrypted by the recipient upon arrival. Most popular email programs have the ability to use a third-party plug-in to provide built-in encryption, including Microsoft® email clients and Mozilla® Thunderbird™.