How to Create an SMTP Server?

Sending email is a critical function for most applications and services. The Simple Mail Transfer Protocol (SMTP) is the standard protocol for sending email messages between servers over the internet. By creating your own SMTP server, you gain more control, security, and customization for your application’s email needs.

How to Create an SMTP Server?

Creating an SMTP server may sound complicated, but with the right tools and understanding, it can be straightforward. In this guide, we’ll cover everything you need to know to create your own SMTP server from scratch.

Overview of the SMTP Protocol

Before diving into creating an SMTP server, let’s briefly overview how SMTP works:

  • SMTP is used for sending email between mail servers, not directly between clients. When you send an email from your email client, it connects to your service provider’s SMTP server which then communicates with the recipient’s SMTP server.
  • It uses TCP port 25 by default to establish connections between mail servers. Mail submissions from clients often use SMTP ports 465 or 587 instead.
  • Communication uses plaintext ASCII text commands. The sending server informs the receiving server who the email is from, who it’s addressed to, and transfers the message content.
  • Once the message transfer completes, the connection closes. SMTP is not designed for long-lived connections.

Understanding these basics will make configuring your own SMTP server much easier.

Choosing an SMTP Server Platform

Many good open source and commercial SMTP platforms exist. Two popular free options include:

  • Postfix – A fast, secure MIME-compliant open source SMTP server for Linux. Postfix is a common default on Linux distros and works well for small to mid-sized servers.
  • hMailServer – Featuring a user-friendly interface, hMailServer is an easy yet full-featured SMTP platform for Windows. It includes useful management tools.

For increased performance and scale, commercial options like Microsoft Exchange or Sendmail are common for larger organizations.

For this guide, we’ll use Postfix on Ubuntu Linux for examples. The principles apply similarly to other platforms though.

Installing the SMTP Server Platform

Once you select your preferred SMTP server software, begin by installing it on your chosen operating system:

On Ubuntu:

Copy code

sudo apt update

sudo apt install postfix

The postfix package provides everything we need to get started.

On Windows (hMailServer):

  1. Download the hMailServer installer
  2. Run the installer, choosing typical options for your environment
  3. Launch the hMailServer Administrator interface

With the SMTP system installed, the real configuration work can begin.

Configuring Your SMTP Server

Configuring a new SMTP server broadly involves these steps:

  1. Updating general server parameters
  2. Setting domains, certificates, and credentials
  3. Integrating with other infrastructure
  4. Testing and troubleshooting

We’ll work through each area to customize Postfix for our needs.

1. Set General Server Parameters

Begin by configuring Postfix general settings for your environment:

  • Edit – Postfix’s core config file defines everything from ports to folders to security policies.
  • Set mail name – Update myhostname to your server’s hostname.
  • List domains – In mydestination, add all hosted domains on this server.
  • Enable networks – Allowed client networks go in mynetworks. Add IP ranges or subnets.
  • TLS encryption – For secure connections, specify certificate and key files in smtpd_tls_cert_file and smtpd_tls_key_file.

Save changes and restart Postfix to apply the new settings. We’ll add more later, but this covers the basics.

2. Configure Domains, Credentials, and Certificates

With connectivity working, we next configure how SMTP handles:

  • Hosted domains – Register domains in /etc/mailname to accept email for additional namespaces.
  • Relay permissions – Allow other servers (like web servers) to relay mail through your SMTP in relay_domains.
  • User authentication – Integrate Postfix with system accounts for credential checking with SASL or Dovecot.
  • TLS certificates – For encryption, install certificates from a trusted CA in the configured key and crt files. Self-signed works internally.

Getting domains, permissions, and credentials properly set ensures smooth email delivery.

3. Integrate with Backend Infrastructure

For actually handling mail, our SMTP server needs to integrate with other services:

  • Mail storage – Use a system like Dovecot, Maildir, or Exchange for user mailboxes. Configure Postfix mail locations in mailbox_command and related variables.
  • Spam filtering – Detect spam with SpamAssassin, greylisting, DNS blocklists, or commercial software like SpamExperts. Enable as middleware transports.
  • Backup – Implement backups for mail storage directories in your usual server backup procedures. Test restores periodically.
  • Monitoring – Track SMTP performance and errors in your monitoring solution using logs, SNMP, or admin interfaces. Get alerts for issues.

Closely integrating Postfix with your infrastructure keeps email flowing smoothly and securely.

4. Test and Troubleshoot Configuration Issues

With configuration complete, thoroughly test your new SMTP server:

  • Internal delivery – Send test messages between local users to confirm basic mailflow. Check syslog logs if issues occur.
  • Remote delivery – Verify you can send and receive external mail by adding an Internet test account.
  • Security scan – Perform scanning like nmap to detect open relays or misconfigurations leaving you vulnerable.
  • Load test – Use utilities like smtp-source to simulate heavy load and ensure your server can keep up. Monitor resource usage.
  • Check spam reputation – At volume, verify major receivers like Gmail don’t blacklist you as a spam source.

Be diligent resolving any issues discovered before going live. A reliable, secure SMTP implementation requires thorough testing.

Operating and Maintaining Your SMTP Server

Once in production, be sure to:

  • Update frequently – Install security patches for your SMTP and OS regularly to prevent vulnerabilities.
  • Renew certificates – Replace expiring TLS certificates to avoid warnings or failed handshakes.
  • Watch queues – If mail queues grow significantly, restart the service and check for bottlenecks like slow recipient domains.
  • Log monitoring – Review logs in a centralized SIEM for signs of compromise like open relays or brute force attacks.
  • User management – Handle new signups, departures, and mail policy changes promptly to prevent access issues.
  • Test backups – Periodically restore from backup to ensure you have working backups of critical mail data.

Proper ongoing maintenance keeps your custom SMTP implementation running smoothly.

Key Takeaways

The key points from creating your own SMTP server include:

  • SMTP provides standardized mail transmission between servers over port 25. Understanding the protocol helps configure your own system.
  • Choose a robust SMTP platform like Postfix or hMailServer appropriate for your scale and tech stack.
  • General parameters, domains, certificates, backups and other infrastructure integrate during configuration.
  • Thoroughly test for functionality, security, deliverability, and performance pre-launch.
  • Update, monitor, manage users, and test backups regularly to operate a reliable custom SMTP service.

With correct configuration tailored to your needs, having your own SMTP server provides security, customization, and control over a critical application function.


Creating an SMTP email server from scratch gives you flexibility and ownership for critical mail delivery needs. Although installation is quick, properly configuring domains, security, backups, and infrastructure integrations takes time. Rigorously testing before launch and maintaining vigilance post-launch ensures your custom SMTP server is safe, reliable, and ready to support your growth. With understanding of key concepts and attention to detail, you can confidently build SMTP capabilities matching the unique needs of your applications.

Frequently Asked Questions

  1. Why create your own SMTP server?
    Owning your SMTP infrastructure allows for greater security, privacy, customization, scale, and cost savings compared to 3rd party services.
  1. What are the alternatives to running my own SMTP server?
    Popular email services like Gmail, Outlook 365, and SendGrid can provide cloud SMTP services instead of managing your own server.
  1. What are the disadvantages of my own SMTP server?
    It requires more effort for security, maintenance, redundancy, troubleshooting and deliverability compared to established cloud options.
  1. What SMTP software options exist besides Postfix and hMailServer?
    Popular alternatives include Sendmail, Qmail, and Microsoft Exchange Server on Windows Server.
  1. Is Postfix or hMailServer more feature rich?
    Postfix is more advanced for enterprise use cases while hMailServer offers an easier interface for small businesses.
  1. What is the easiest OS choice for running Postfix?
    Linux distributions like Ubuntu, Debian, CentOS, and RHEL include Postfix in repositories for simplified installs.
  1. What permissions does Postfix require?
    Postfix runs as a dedicated user like postfix and requires typical folder permissions to mail directories like /var/spool.
  1. What backend options exist for user mail storage?
    Dovecot, Maildir, MySQL, and direct NFS/SMB mounts are common Postfix mailbox formats.
  1. What is the best practice for encrypting SMTP connections?
    Use trusted TLS certificates allows for authenticated secure connections protecting message content.
  1. What SMTP port numbers are commonly used?
    Standard SMTP uses port 25. Submissions support SSL on ports 465 or 587. Non-standard ports may also appear.
  1. What risks are introduced by allowing mail relaying?
    Open relays mean external entities can route spam or malicious emails through your server. Restrict relaying to only authorized servers.
  1. How can you validate Postfix configurations before launch?
    Test mailing between local users, to Internet accounts, perform security scans, check spam blacklists, and load test the server.
  1. What maintenance best practices are critical for SMTP servers?
    Apply security patches, renew certificates, monitor queues, create backups, monitor logs, and test restores regularly.
  1. How can you detect compromised or improperly configured SMTP servers?
    Monitor rejected IPs, watch inbound connection counts, review authentication failures, check reputation sites, and validate config files.
  1. What are common Postfix troubleshooting techniques?
    Check Postfix and system logs, trace mailflow with SMTP debugging, test individual configuration parameters in isolation, and monitor server resource metrics.
  1. What causes ‘too many errors’ alerts in Postfix?
    These often indicate connectivity issues reaching recipient servers. Check DNS, firewalls, recent configuration changes, and the target domain’s mail infrastructure.
  1. How do you resolve messages stuck in the mail queue?
    If the queue grows significantly, restart the Postfix service. Also check for rate limiting or greylisting on receiving servers.
  1. What best practices help improve email deliverability?
    Proper configuration, DNS setup, using whitelist providers, warming IP address reputation, and double opt-in confirmation messaging when applicable.
  1. Where can you learn more about Postfix configuration options?
    The official Postfix documentation provides extensive context and examples for all configuration parameters and customization.
  1. What security hardening steps help protect SMTP servers?
    Minimize exposed network surface area, enable firewall rules restricting access, disable unnecessary services, apply OS and software patches, and use encrypted connections.

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