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SMTP – Simple Mail Transfer Protocol

However, SMTP is a communication procedure for the transmission of emails. The message occurs between a mail client and an SMTP server (outgoing mail server) or between two SMTP servers.

Mail Transfer Agents (MTAs) are responsible for the exchange of emails. MTAs communicate with each other using the SMTP protocol.

Besides SMTP, there are two other email exchange protocols: POP and IMAP. However, these two protocols are only recycled to collect emails or manage them online. SMTP, on the added hand, is a communication protocol that can receive and forward emails.

Email routing via SMTP and DNS

Once the SMTP server receives an email from an email client (SMTP client), it is responsible for forwarding it to the destination SMTP server. As with the HTTP and FTP protocols, DNS plays a central role. There are notable entries in DNS for email. These are the mail exchange records (MX records). The SMTP server uses these entries to identify the destination SMTP server for the domain specified in the recipient email address.

The email routing process looks like this: The SMTP server queries a DNS server and gets a list of mail servers receiving email for the destination SMTP server. Each of these mail servers (Mail Exchange) is assigned a priority. The SMTP server tries to contact the mail server in the order specified to forward the email.

In theory, an email may pass through more than one of these mail exchangers. MX records intend to prevent these mail loops from occurring. However, mail loops can occur if MX records are incomplete or the domain has moved to another host or provider.

Disadvantages of SMTP

SMTP has several significant drawbacks. On the one hand, no shipping confirmation return for sent emails. If an email is lost, neither the sender nor the recipient will remain notified.

If an email cannot be delivered, the SMTP specification provides for notification of the sender. There is an SMTP extension for standardized error messages, but not all SMTP servers support this extension. Most undeliverable emails contain only a more or less understandable error message in English, and the header of the email sent.

Another problem with SMTP is the lack of user authentication when establishing a connection between the SMTP client and the SMTP server. It means that any sender address can be specified when sending an email. In practice, it appears that masses of advertising emails, known as spam, are sent through open SMTP servers. Due to spoofed sender addresses, the actual sender can only stay determined with great effort.

Over time, measures and procedures have developed to combat the misuse of SMTP servers. Unfortunately, these procedures are optional. That means that it is up to the administrators to activate and configure the systems in their mail servers.

Structure of an email

An email has three parts. The envelope contains the return address and recipient address that the MTA requires. That follow by the header with information about the email client and the message ID. It is a number/letter combination followed by the host address (domain) of the sender. The body contains the text of the email message.