Sendmail is a server application that gives businesses a way to send email using the Simple Mail Transfer Protocol (SMTP). It’s typically installed on an email server on a dedicated machine that accepts outgoing email messages and then sends these messages to the defined recipient. It queues messages if a recipient is not immediately available and offers authentication as a method to prevent spam. Sendmail, however, only sends email and does not have Post Office Protocol (POP) or Internet Message Access Protocol (IMAP), allowing it to receive messages and store them in user inboxes. Therefore, Sendmail is typically installed with additional applications that enable user inbox configurations.
How Sendmail Works
It’s crucial to separate SMTP from the Sendmail server application to understand how it works. Sendmail is an application that includes SMTP functionality and configurations, but SMTP is the protocol used to send email messages. Cloud hosts offer a commercial application named Sendmail that can be used to send email via an application on a Windows or Linux server.
SMTP in Sendmail can be used with Secure Socket Layer (SSL) to keep messages secure from man-in-the-middle (MitM) attacks. Mail protocols are inherently insecure, but using SSL and authentication stops attackers from reading messages and spammers from using a business email server for spam messages.
When an SMTP server with Sendmail receives an outbound message, it performs two actions and generates two files for each one. The first action is to identify recipient email addresses and package them into a header. The header file contains a list of recipients, including aliases and mailing lists. It also checks the syntax of the outgoing message to ensure that the message is sent correctly.
The second action creates a file containing the message body. Sendmail takes each recipient address and attaches them to the body and header file and then sends the message to the specified recipient. The outgoing mail server is unaware of the recipient's existence, so any issues with the message such as an incorrect email account will result in the message being returned. Email headers are essential in transferring messages, as the header provides the recipient with information about its path and authenticity.
Sendmail has a few error-handling features. For instance, if the exit status returns “temporary failure,” the application will queue the message to try again later. If the application is unable to send the message, a failure message is returned to the sender to let them know that the message was never sent.
Certain sections of email headers are unique information added by an organization, but other parts are always included when using Sendmail. The path a message takes to get to the recipient server is always included in a message header, but organizations can choose to add their own header values provided it’s in the right syntax. The following is the basic syntax of an email header value:
Field name: value
The field name-value pair must be on its own line, or the syntax validation step could fail, and outgoing messages will never be sent. Several security standards have been created for email to stop spoofing and phishing. These security headers are also appended to outgoing messages to validate that the sender email address is legitimate and not spoofed.
How to Use Sendmail
The Sendmail commercial application can be installed on Windows or Linux, but the Sendmail package that contains SMTP is built for the Linux operating system. Cloud providers offer automated installation of the application making it easy for businesses and their administrators to set up an outgoing email server without knowing the configurations needed to set it up properly.
With a graphical user interface, users do not need to use the command line to send an email. A Sendmail-installed server can also handle outgoing email messages from a web application. For businesses with virtual machines and their own web servers, Sendmail must be installed on the server, but most web hosts include the installation in cPanel. Any automated installations do not require manual configurations.
Sendmail is a backend service, so its operations are invisible to an end-user. Should an administrator decide to test Sendmail from the command line, several options and commands are available, but the basic syntax to send a simple message as follows:
Typing the above command activates Sendmail so that an email can be sent to the given address. As an example, the following input sends an email from firstname.lastname@example.org with the message “Hello” in the body:
From: email@example.com To: firstname.lastname@example.org Hello .
Notice the “.” character on a separate line at the end of the message. Think of the “.” character as the “end” flag, telling Sendmail that the message is finished. Sendmail takes all information above the “end of message” character and sends the email message. If any syntax errors are found, Sendmail returns an error to the administrator.
Command-line statements are usually employed for testing Sendmail, so it’s unusual for a user ever to need to apply them. Instead, any email client installed on a user’s device can be used to work with Sendmail. The email client application (e.g., Outlook, Thunderbird) must be configured to connect to the Sendmail server using the SMTP settings. After the client application is set up, users send messages without ever working directly with the Sendmail commands.
Benefits of Sendmail
The overall benefits of using Sendmail are tied to having an internal email server. Cloud email servers are an option, but some businesses need the security of hosting an email server internally. This gives companies the ability to customize email headers and server behaviors without relying on the automatic configurations of a cloud host.
Another benefit of hosting your own email server is that it can more easily integrate with internal applications. Developers do not need to verify that the external email servers that are coded into the application are insecure. It also ensures that the email server is always available, and configuration changes can be tested before they change without notice.
The benefits of the Sendmail application are similar to other mail applications. It’s fully configurable, can be remotely managed by an administrator, it provides failure messages to users when message transfers fail, and it can resolve names before sending messages. These benefits offer administrators and organizations a reliable way to host an email server. The application itself is lightweight, so it doesn't require a lot of server resources to run the software.
Maintaining a Sendmail server is an added task for administrators, so businesses should ensure that their IT staff has the time and skillset to maintain an email server. Although Sendmail is lightweight and easily configurable, it still must be maintained and monitored. Sendmail offers simple monitoring tools, but the new infrastructure component will add risk to an organization’s overall threat management procedures.