How to Build a Mail Server

This document provides a detailed guide on how to build a custom SMTP mail server. It covers steps such as choosing a server and operating system, installing and configuring SMTP server software, setting up DNS records, securing the mail server, testing the SMTP server, installing anti-spam and anti-virus software, regular maintenance, and backup and disaster recovery. It also includes additional tips and answers common questions related to firewall rules, testing mail delivery, altering the path where emails are stored, and handling emails for a specific user locally.

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Building a custom SMTP mail server involves several steps, starting from setting up the server to configuring the SMTP software and securing the server. Here's a detailed guide:

1. Choose a Server and Operating System

  • Select Hardware: Based on your expected email volume and performance requirements.
  • Choose an Operating System: Linux is widely preferred for its stability and security. Popular choices include Ubuntu, CentOS, and Debian.

2. Install and Configure SMTP Server Software

  • Install SMTP Software: Postfix and Exim are popular SMTP servers for Linux. You can install them using your Linux distribution’s package manager. For example, on Ubuntu, use sudo apt-get install postfix.
  • Basic Configuration: During installation, you'll be prompted to configure some basic settings like the domain name and type of mail configuration (Internet Site, Satellite system, etc.). Choose according to your needs.
  • Advanced Configuration: After installation, edit the configuration files (/etc/postfix/main.cf for Postfix) to set up:
    • Hostname
    • Domain name
    • Network interfaces to listen on
    • Mail relay settings, if applicable

3. Set Up DNS Records

  • MX Record: Create an MX record in your DNS settings pointing to your mail server.
  • A Record: Ensure there's an A record in DNS pointing to your server's IP address.
  • SPF, DKIM, and DMARC Records: Set up these DNS records for email authentication and to improve deliverability.

4. Secure Your Mail Server

  • Implement TLS Encryption: To encrypt emails in transit, configure your SMTP server to use TLS. This often involves obtaining a TLS certificate from a certificate authority and configuring your SMTP server to use this certificate.
  • Configure Firewall: Open the necessary ports (usually port 25 for SMTP) and close all unnecessary ports.
  • Use Authentication: Require username and password authentication for sending emails.
  • Implement Rate Limiting and Size Limits: To protect against spam and abuse.

5. Test Your SMTP Server

  • Send Test Emails: Use a tool like swaks (SMTP test tool) or a script to send test emails to verify that your server is sending emails correctly.
  • Check Deliverability: Make sure your emails are not being marked as spam by recipient servers.

6. Install Anti-Spam and Anti-Virus Software

  • Use tools like SpamAssassin for spam filtering.
  • Install ClamAV for virus scanning.

7. Regular Maintenance

  • Monitor Server Logs: Keep an eye on /var/log/mail.log for any unusual activity.
  • Keep Software Updated: Regularly update your operating system and mail server software.

8. Backup and Disaster Recovery

  • Regularly backup your server configuration and email data.
  • Have a disaster recovery plan in place.

Additional Tips

  • Testing DKIM and SPF: Use online tools to test that your DKIM and SPF settings are correct.
  • Reverse DNS: Set up reverse DNS for your server's IP address for better email deliverability.
  • Capacity Planning: Monitor your server's capacity and plan for scaling as your email volume grows.
Building and maintaining a mail server is a complex task that requires a good understanding of mail protocols, network security, and system administration. If you are new to this, consider starting with simpler setups and gradually increasing complexity, or consult with IT professionals.

Do I need to add pass rules for the default port 25?

Yes, if you are setting up an SMTP mail server, you need to ensure that your firewall allows traffic through the default SMTP port, which is port 25. This is essential for your mail server to send and receive emails. Here’s how you can manage firewall settings for this:

For Linux Servers with iptables

  1. Check Existing iptables Rules: Before modifying, check your existing rules with:
    1. Add Rules to Allow Port 25:
        2. These commands add rules to accept incoming and outgoing traffic on port 25.
    1. Save the iptables Rules: Depending on your distribution, the command to save iptables rules varies. For example, on Ubuntu, you might use iptables-save.

    For Servers with firewalld (like CentOS)

    1. Check Current Zones:
      1. Add Port 25 to the Public Zone (or your relevant zone):
        1. Reload firewalld to Apply Changes:

          For Servers Using UFW (Uncomplicated Firewall)

          1. Allow Port 25:
            1. Enable the Changes:

              Important Considerations

              • Security: While port 25 is necessary for SMTP, it's also known for being a vector for spam and malicious activity. Ensure you have security measures like spam filtering and monitoring in place.
              • ISP Restrictions: Some ISPs block port 25 to reduce spam. If you're setting this up on a residential connection, check with your ISP. You might need to use a different port for outgoing mail (like port 587).
              • Use Port 587 for Submission: Modern email practices recommend using port 587 for email submission (sending out emails from email clients). Ensure this port is also open and properly configured in your mail server settings.
              Remember to always verify your firewall settings to ensure they're correctly configured and not conflicting with any other rules. This is crucial for both the functionality of your mail server and the overall security of your system.

              How to change firewall rules in UFW

              To change firewall rules in UFW (Uncomplicated Firewall) to allow UDP traffic on port 80, you'll need to use the UFW command-line interface. The rule you've mentioned is more like an iptables rule, but UFW simplifies this process.
              Here's how you can add this rule with UFW:
              1. Open Terminal or Access Your Server: If you're not already, open a terminal window or SSH into your server.
              1. Allow UDP Traffic on Port 80: Use the following command to allow UDP traffic on port 80:
                  2. This command updates the UFW rules to allow incoming UDP packets on port 80.
              1. Check the Rules: After adding the rule, you can check the status of UFW to see all the active rules:
                  2. This will display a list of all rules, including the one you just added.
              1. Reload UFW: If you need to apply the changes immediately, you can reload UFW:
                  2. This step is usually not necessary as UFW applies changes immediately after a rule is added.

              Important Notes:

              • Port 80 for UDP: Port 80 is traditionally used for HTTP traffic, which is TCP-based. Allowing UDP traffic on port 80 is unusual and might be necessary only for specific applications or setups. Make sure this is what you need for your configuration.
              • Security Considerations: Be cautious when opening ports, especially common ports like 80. Ensure this change aligns with your server's security policy and doesn't expose your system to unnecessary risks.
              • UFW vs. iptables: UFW is a front-end for iptables and is designed to simplify firewall configuration. If you're more comfortable with iptables, you can also use that, but UFW is generally easier for basic to moderate configurations.
              Before making changes to your firewall configuration, ensure you understand the implications, and it's always a good idea to have backups of your current configurations.

              How to unset the rules for 25 port

              To remove or unset firewall rules for port 25, which is typically used for SMTP traffic, you'll need to use commands specific to the firewall software you're using. I'll cover how to do this for the most common Linux firewall tools: iptables, firewalld, and UFW (Uncomplicated Firewall).

              For UFW (Uncomplicated Firewall)

              1. Remove Rule:
                  2. This command removes the rule that allows traffic on port 25.
              1. Check the Status:
                  2. Verify that the rule has been removed.

              For iptables

              1. List Current Rules with Line Numbers:
                  2. Identify the line number for the rule that allows port 25.
              1. Delete Rule by Line Number: Assuming the rule is on line number X:
                  2. Replace X with the actual line number of the rule.
              1. Save Changes: Depending on your distribution, you might need to save the iptables configuration to make it persistent. For example, on Debian/Ubuntu:

                For firewalld

                1. Remove Rule: First, identify the zone where the rule is applied (e.g., public):
                  1. Reload firewalld:

                    Additional Notes

                    • Test Your Configuration: After changing firewall settings, it's essential to test your configuration to ensure it behaves as expected.
                    • Understand the Impact: Be aware that removing rules for port 25 will stop your server from sending and receiving SMTP traffic, which could impact email services running on the server.
                    • Persistent Changes: For both iptables and firewalld, make sure changes are made persistent (if your system requires it) so that they survive a reboot.
                    • Backup: It's always a good practice to backup your current firewall configuration before making changes.
                    Removing a firewall rule is a straightforward process, but it's important to understand the implications of doing so, especially for critical services like email.

                    How to unset this command: sudo iptables -A INPUT -p tcp --dport 25 -j ACCEPT

                    To unset or remove a rule you've added with iptables, such as sudo iptables -A INPUT -p tcp --dport 25 -j ACCEPT, you'll essentially need to issue a command that does the opposite. In the case of iptables, this means deleting the specific rule.
                    Here's how you can do it:

                    Step 1: List the Current iptables Rules with Line Numbers

                    First, list out the current rules along with their line numbers in the INPUT chain, as this will help you identify the exact rule to remove:
                    Look for the line that corresponds to the rule you want to remove, such as:
                    In this example, let's assume the rule you want to delete is number 2.

                    Step 2: Delete the Rule by Line Number

                    Use the following command to delete the rule, replacing 2 with the actual line number from your output:

                    Step 3: Save the Changes

                    Depending on your Linux distribution, the changes might not be automatically saved. If you're using a system with iptables-persistent or a similar service, you'll need to save these changes:
                    • On Debian/Ubuntu:
                      • On CentOS/RHEL (if you have installed iptables-services):

                        Important Notes

                        • Impact of Removing the Rule: Removing this rule will stop your server from accepting incoming SMTP traffic on port 25. Ensure this aligns with your server's intended use and security policy.
                        • Recheck the Firewall Configuration: After making changes, always verify your firewall setup with sudo iptables -L to make sure it reflects your intentions.
                        • Persistent Changes: Make sure that the changes are persistent across reboots, as mentioned above.
                        Remember, managing iptables requires precision, as a small mistake could lock you out of your server or expose it to security risks. Always double-check the rules and their implications before applying them.

                        Use Postfix's sendmail utility to send test mail

                        To send a test email using Postfix's sendmail utility, follow these steps. This method works on a Linux server where Postfix is installed and configured:

                        1. Compose the Email

                        First, create a text file that contains your email. Open a text editor (like nano or vim) and compose your email in the following format:
                        Save this file with a name like testemail.txt.

                        2. Send the Email using sendmail

                        Now, use the sendmail command to send your email. You can do this by piping the content of your text file into the sendmail command. In your terminal, navigate to the directory where you saved testemail.txt and run:
                        The -t option tells sendmail to look for To, Cc, and Bcc headers in the message to determine the recipients.

                        3. Check Delivery

                        After sending the email:
                        • If it hasn’t arrived, check the spam/junk folder.
                        • You can also check your mail server logs for any error messages or delivery issues. On most Linux systems with Postfix, the log file is located at /var/log/mail.log or /var/log/maillog.

                        Troubleshooting

                        If the email does not arrive:
                        • Verify that Postfix is running: sudo systemctl status postfix.
                        • Check for errors in the Postfix log files (/var/log/mail.log or /var/log/maillog).
                        • Ensure that your DNS and MX records are properly configured.
                        • Make sure there are no firewall or network issues preventing email delivery.

                        Note

                        Using sendmail directly like this is fine for test emails or occasional use, but for sending emails programmatically or in larger volumes, it's advisable to use a proper email library or tool that can handle complexities like MIME types, attachments, and encoding.

                        How to alter the path where emails are stored for Postfix?

                        To alter the path where emails are stored for Postfix, you will need to modify the configuration to specify a different location for the mailbox. This can be done in the main.cf configuration file of Postfix. Whether you are using the mbox or Maildir format, the process involves specifying a different path for the mailbox or mail directory.

                        For Maildir Format

                        If you're using the Maildir format, emails are stored in a directory structure in each user's home directory by default. To change this:
                        1. Open the Postfix Configuration File:
                          1. Set the home_mailbox Parameter:
                              • Add or modify the home_mailbox line to specify the new path. For example, to change the mail directory to a folder named "Email" in each user's home directory, you would add:
                                • Ensure the path ends with a slash (/) to indicate that Maildir format is being used.
                            1. Save and Close the File.
                            1. Restart Postfix:

                              For mbox Format

                              If using the mbox format, where all mails are stored in a single file, you will modify the mailbox_command parameter:
                              1. Open the Postfix Configuration File:
                                1. Set the mailbox_command Parameter:
                                    • Modify this parameter to specify a command for delivering mail to a custom location. For example:
                                      • In this case, you would configure procmail (or another mail delivery agent) to deliver mail to the desired location.
                                  1. Save and Close the File.
                                  1. Restart Postfix:

                                    Important Notes

                                    • Changing the mail storage path can disrupt mail delivery if not done correctly. Make sure you understand the implications and have backups before making changes.
                                    • If you change the mail storage format or path, you'll need to inform users, as they may need to update their email client settings.
                                    • After making changes, test to ensure that mail delivery works as expected.
                                    • For changes involving procmail or another delivery agent, ensure that the corresponding software is installed and properly configured.
                                    Always back up your current configuration before making such changes, and test thoroughly after applying them.

                                    How to handle the emails for [email protected] locally for root user?

                                    To handle emails for [email protected] locally and deliver them to the root user on your Postfix server, you need to set up an alias in the /etc/aliases file. This file allows you to redirect mail intended for one address to another. Here's how you can do it:

                                    Step 1: Edit the /etc/aliases File

                                    1. Open the Aliases File: Use a text editor like nano or vim to edit the /etc/aliases file. You might need superuser privileges for this:
                                      1. Add an Alias for [email protected]: In the /etc/aliases file, add a line that specifies [email protected] should be redirected to the root user. It should look like this:
                                          2. This means any email sent to [email protected] will be delivered to the root user's mailbox.
                                      1. Save and Exit: Save the file and exit the text editor.

                                      Step 2: Update the Alias Database

                                      After modifying the /etc/aliases file, you need to build the new aliases database. This is done with the newaliases command:

                                      Step 3: Restart Postfix

                                      It's generally a good practice to restart the Postfix service after making configuration changes:

                                      Step 4: Test the Configuration

                                      Send a test email to [email protected] and then check the root user's mail to see if it has been received. You can check the root user's mail using a command like:
                                      or by checking the root user's mail file directly (e.g., /var/mail/root or /root/Maildir depending on your configuration).

                                      Important Notes

                                      • Security Consideration: Redirecting emails to the root user should be done cautiously, as it can have security implications. The root mailbox can contain sensitive system information.
                                      • Mail Access: Accessing the root mailbox requires superuser privileges.
                                      • Mail Client Configuration: If you're using a mail client to read the root user's mail, configure it with the appropriate permissions and settings.
                                      This setup will allow you to handle emails for [email protected] locally and deliver them to the root user's mailbox on your Postfix server.
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