You do not have any new mail

Linux users which run Debian and recently upgraded from Debian 12 to 13 (Trixie) might have been greeted by You do not have any new mail on their first log in on a TTY. Well if the user also has a mail server installed I suppose.

The message "You do not have any new mail" was at least confusing for me. Until Debian 12 you got You have mail when read mail exist or You have new mail if there are unread mails in /var/mail/$USER, with $USER being the name of the user currently logged it. When I first read the new message "You do not have any new mail" I thought "I have new mail, how come?", because it just directly after the boot completing the system upgrade. I had to read two or three times to "Hum, that is odd". Frankly I have no idea why this message comes, or why somebody deemed it necessary to change the old behaviour.

For those not having a local mail system installed, here are some infos which might help to understand its benefits.

Introduction to the Linux Mail System

The local mail system in Linux is part of almost all Unix-like operating systems. It is designed to enable users and system processes to exchange messages from and to one another user on the same machine. It is also possible to use SMTP Servers like the one from Google's Gmail, Yahoo, Hotmail and others to deliver mail to the outside. To deliver mail via a local mail server like Gmail, with for example Postfix, SMTP AUTH needs to be set up by altering the main.cf file and creating or editing the /etc/postfix/sasl_password file.

No GUI needed

Long before graphical interfaces, text messaging and social media platforms local email was the primary way the system communicated information such as error reports, job completion notices or administrative alerts. Local mail remains critical today for system administration, automation, and logging, especially on servers and headless systems. These systems doesn't even need to have a GUI or mouse support installed.

At its core, the Linux mail system is modular. Instead of being a single monolithic program, it is composed of several cooperating components, each responsible for a specific part of mail handling. These components follow long-established Unix design principles: small tools that do one job well and interact through well-defined interfaces.

Linux Mail core components

The local mail system is typically described in terms of three major components: the Mail User Agent (MUA), the Mail Transfer Agent (MTA), and the Mail Delivery Agent (MDA). While these roles can sometimes be combined into a single program, conceptually they remain distinct and help explain how mail flows through the system.

The Mail User Agent (MUA) is the interface through which users read and send mail. Examples include command-line tools such as mail and mutt. When a user sends a message using an MUA, the message is handed off to the local mail system for delivery. For local mail, the recipient is usually another user account on the same machine.

The Mail Transfer Agent (MTA) is responsible for routing and transferring mail. Common MTAs on Linux systems include sendmail and Postfix, as well as Exim.

The Mail Delivery Agent (MDA) handles the final step: delivering the message into the recipient’s mailbox. Examples include procmail, but is usually substituted by the built-in local delivery mechanisms of modern MTAs like Postfix. The MDA decides where and how the message is stored and may apply filtering rules before delivery.

Mail flow on a local system

When a user or system process sends a message to a local user, the flow is straightforward. The MUA generates the message and submits it to the MTA, typically using the local sendmail interface or SMTP over the loopback interface. The MTA inspects the recipient address and recognizes it as local, meaning it corresponds to a user account on the system.

Once identified as local mail, the MTA invokes the MDA to perform delivery. The MDA then writes the message into the recipient’s mailbox file or directory. The recipient can later read the message using their preferred MUA. From the user’s perspective, this entire process is usually invisible and happens almost instantly.

Mailbox formats and storage

Linux systems support multiple mailbox formats. Traditionally, mail was stored in a single file per user, often located in /var/mail/username or /var/spool/mail/username. This format, known as mbox, appends each new message to the end of the file. While simple, mbox files can grow large and require careful file locking to avoid corruption.

A more modern approach is the Maildir format, which stores each message as a separate file within a directory structure. Maildir improves reliability and performance, especially under concurrent access, and is commonly used by modern MTAs and MDAs. Whether mbox or Maildir is used depends on system configuration and administrator preference.

Local mail and system automation

One of the most important uses of local mail is system automation. Tools like cron, to a lesser degree at and mechanisms provides by systemd automatically send mail containing the output of scheduled jobs. If a cron job produces output or encounters an error, the results are emailed to the job’s owner or a configured address. This makes local mail an essential monitoring and debugging tool.

Similarly, backup scripts, log rotation jobs, and security tools frequently rely on local mail to notify administrators of success or failure. Even on systems that do not handle external email, local mail remains indispensable for internal communication.

Conclusion

Understanding how local mail works gives administrators better insight into system behavior and troubleshooting. It also highlights the enduring strengths of Unix design: simplicity, modularity, and powerful composition of small tools working together.

Built around clear roles—MUA, MTA, and MDA—it provides reliable communication between users and system processes. Even in an era dominated by instant messaging and web-based notifications, local mail continues to play a crucial role in system administration, automation, and reliability, albeit with diminishing importance.

So if you run Debian 13 (Trixie) or later and have a local mail server installed like described above, chances are you'll also see the You do not have any new mail welcome message when you logged in to a TTY, unless your system or one of your users sent you a mail. Then you'll see You have mail. Note that you will not see this message if you start an X-Terminal (or Xterm for short).