What is SMTP? Defin­i­tion and basics

If you think of e-mail as elec­tron­ic mail, then SMTP is the mail man. Without the internet’s standard network protocol, sending ap­plic­a­tions documents, e-cards, and funny cat videos wouldn’t be possible at all. This is because when you send an e-mail, it doesn’t simply travel directly to the recipient, but goes through a complex com­mu­nic­a­tion process with numerous instances before it reaches its des­tin­a­tion.

SMTP stands for “Simple Mail Transfer Protocol.” It is a con­nec­tion-ori­ent­ated, text-based network protocol from the internet protocol family and is located on the seventh layer of the OSI model: the ap­plic­a­tion layer. Just like any other network protocol, it contains rules for correct com­mu­nic­a­tion between computers in a network. SMTP is re­spons­ible for feeding and for­ward­ing e-mails from sender to recipient.

Since its release in 1982 as the successor of the “Mail Box Protocol” in Arpanet, SMTP has become the standard protocol for sending e-mails. For the average consumer, however, the SMTP protocol remains largely invisible, since the re­spect­ive e-mail program executes it in the back­ground. If the software, the webmail ap­plic­a­tion in the browser, or the mobile mail app don’t auto­mat­ic­ally define the SMTP protocol when setting up an account, you have to set it up manually to ensure smooth e-mail traffic.

Here is an overview of SMTP server addresses and SMTP ports for some of the most popular e-mail providers.

The in­ter­ac­tion between the SMTP client and the SMTP server mentioned in the first step of the SMTP procedure rep­res­ents the actual SMTP session. The rest of the SMTP procedure i.e. for­ward­ing the e-mail via SMTP from the server to the recipient is covered in a separate article. Each session consists of a sequence of SMTP commands from the client and responses in the form of status codes from the server.

According to the ap­plic­able SMTP spe­cific­a­tions, each im­ple­ment­a­tion of the network protocol must support at least the following eight commands, con­sist­ing of 7-bit ASCII char­ac­ters:

The server responds to each of these SMTP commands of the client with a three-digit status code including plain text message:

The dis­ad­vant­age of SMTP is that users are not verified when a con­nec­tion is es­tab­lished, meaning that the sender of an e-mail might not be trust­worthy. As a result, open SMTP relays are often used to send spam on a massive scale. The spammers use fake sender addresses so that they cannot be traced (mail spoofing). Today, there are many security tech­no­lo­gies in use to prevent SMTP servers from being abused. For example, sus­pi­cious e-mails are rejected or moved to quar­ant­ine (spam folder). The iden­ti­fic­a­tion protocol Do­main­Keys (DK), the Sender Policy Framework (SPF), or greylist­ing can be used for this purpose. In addition, it has become more or less standard to receive e-mails not only via the tra­di­tion­al port 25/TCP, but also via port 587/TCP, so that trans­mis­sion is only allowed for verified users.

In response to the rampant spam problem on the internet, an extension of SMTP was released in 1995: extended SMTP (ESMTP for short). It adds ad­di­tion­al commands to the protocol in 8-bit ASCII code, enabling many new functions to save bandwidth and protect servers. These include, for example:

• Au­then­tic­a­tion of the sender
• SSL en­cryp­tion of e-mails
• Pos­sib­il­ity of attaching mul­ti­me­dia files to e-mails
• Re­stric­tions on the size of e-mails according to server spe­cific­a­tions
• Sim­ul­tan­eous trans­mis­sion to several re­cip­i­ents
• Stand­ard­ised error messages in case of un­deliv­er­ab­il­ity

Lastly, here are some examples of ESMTP commands: