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-oriented, 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
• Standardized error messages in case of undeliverability

Lastly, here are some examples of ESMTP commands: