Next-generation, programmable MTA

State-of-the-art SMTP server built for unmatched security, enhanced speed, superior scalability, and extensive customization capabilities.

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Sender & Message Authentication

Comprehensive sender and message authentication.

Stalwart supports the latest standards in sender and message authentication, including DMARC, SPF, DKIM, and ARC. This comprehensive support ensures that every email sent and received is verified for authenticity, enhancing security and trustworthiness. Stalwart also excels in proactive communication with other servers by sending and analyzing authentication reports. This automatic exchange and examination of reports allows Stalwart to identify and alert administrators of potential issues promptly, providing an additional layer of security and maintaining the integrity of your email communications.

  • Domain-based Message Authentication, Reporting, and Conformance (DMARC) verification and reporting.
  • DomainKeys Identified Mail (DKIM) verification, signing and failure reporting.
  • Sender Policy Framework (SPF) policy evaluation and failure reporting.
  • Authenticated Received Chain (ARC) verification and sealing.
  • Reverse IP (iprev) validation.
  • Automatic analysis of received authentication failure and abuse reports.

Strong transport security

Robust email security at the transport layer.

Stalwart incorporates robust transport security features designed to safeguard email communications at the transport layer. By utilizing protocols such as DANE and MTA-STS, Stalwart ensures that email data remains secure during transit, preventing interception and tampering. Additionally, the platform automatically analyzes incoming TLS reports, enhancing its ability to identify and react to potential security threats. This proactive approach to security not only maintains the confidentiality and integrity of email data but also fortifies the email infrastructure against evolving cyber threats.

  • DNS-Based Authentication of Named Entities (DANE) Transport Layer Security.
  • SMTP MTA Strict Transport Security (MTA-STS).
  • SMTP TLS Reporting (TLSRPT) delivery and analysis.
  • Automatic analysis of incoming TLS aggregate reports.

Advanced filtering

Enhanced filtering and rewriting.

Stalwart is equipped with a comprehensive set of filtering capabilities designed to enhance email security and management. Its robust spam and phishing filters effectively block malicious and unwanted content, maintaining the integrity of your communications. Additionally, the platform's support for the Sieve scripting language allows for intricate customization of filtering rules. Stalwart also incorporates advanced control features such as inbound concurrency and rate limiting to manage and optimize the flow of incoming emails, ensuring efficient and secure email handling.

  • Built-in SPAM and Phishing filtering.
  • Sieve scripting with support for all registered extensions.
  • Milter support for integration with external content filtering systems.
  • Filtering, rewriting, modification and removal of message parts, envelope or headers.
  • Inbound concurrency & rate limiting.

Flexible queues

Advanced queue management for efficient delivery.

Stalwart enhances SMTP operations with sophisticated message queueing features designed to optimize and secure email delivery. The system utilizes distributed and fault-tolerant queues that ensure reliable message handling across diverse network conditions. With support for unlimited virtual queues, dynamic routing rules, and specialized delivery options, Stalwart can meet varied organizational needs. Additionally, it helps manage the flow and volume of outgoing messages, preventing bottlenecks and maintaining system efficiency.

  • Distributed and fault-tolerant message queues.
  • Unlimited virtual queues.
  • Dynamic routing rules.
  • Delayed & priority delivery support.
  • Outbound throttling.
  • Queue quotas.


Programmable MTA for modern email systems.

Stalwart stands out for its exceptional flexibility and programmability, tailored to meet the unique demands of any modern messaging infrastructure. It allows for extensive customization through dynamic configuration rules, enabling administrators to fine-tune behaviors and settings in real time. Additionally, the message queue is highly adaptable, supported by a wide range of storage backends. This versatility ensures that regardless of the scale or complexity of your needs, Stalwart can be configured to deliver optimal performance and reliability.

  • Programmable with dynamic configuration rules.
  • FoundationDB, RocksDB, Redis, LDAP, PostgreSQL, MySQL, and SQLite support.
  • OpenTelemetry support for monitoring, tracing, and performance analysis.
  • REST API for management.

Web-based admin

Real-time web-based MTA monitoring.

Stalwart offers comprehensive web-based management capabilities, allowing for complete control of the email queue directly from your browser. Its webadmin tool simplifies the administration of email queues and provides detailed insights into DMARC, TLS-RPT, and ARF reports, enhancing visibility into email performance and security. Developed as a Single Page Application (SPA), the webadmin interface is exceptionally responsive and fast, ensuring a smooth and efficient management experience.

  • Web-based queue management for messages and outbound DMARC and TLS reports.
  • Report visualization interface for received DMARC, TLS-RPT and ARF reports.
  • Configuration of every aspect of the SMTP server.
  • Log viewer with search and filtering capabilities.

High availability

Seamless failover with distributed queues.

With its advanced distributed message queue system, Stalwart is engineered for high availability. Unlike other MTAs that rely on local storage for message queues, Stalwart utilizes a distributed approach. This design allows any node within the cluster to seamlessly take over message delivery duties if another node fails, ensuring no interruption in service. Such redundancy guarantees that all messages will be reliably received, maintaining continuous communication even in the face of hardware or network failures. This robust framework provides an extra layer of reliability and efficiency, making Stalwart an ideal choice for businesses that cannot afford any downtime.

  • Distributed and fault tolerant SMTP queue.
  • Node autodiscovery, streamlining network expansion.
  • Network partition-tolerant failure detection.
  • Simplified architecture without coordinators or proxies.