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Core Architecture

LSMTPD is designed to be lightweight and fast, with a focus on simplicity and performance.
Its architecture follows a modular design, making it both extensible and easy to reason about.

Core Components​

1. Connection Handling​

  • LSMTPD efficiently manages incoming SMTP client connections.
  • Each connection is handled asynchronously using tokio::spawn, allowing thousands of concurrent sessions without blocking.
  • A fixed 180-second timeout ensures idle or stuck connections are closed gracefully.

2. Command Processing​

LSMTPD parses and responds to standard SMTP commands using a simple state machine.
Supported commands include:

  • EHLO / HELO → Greet the server.
  • MAIL FROM → Define the sender.
  • RCPT TO → Add recipients.
  • DATA → Begin message body transfer.
  • . (Dot) → End of message body.
  • QUIT → Close the connection.
  • RSET → Reset the current transaction.

This minimal but standards-compliant handling ensures interoperability with typical SMTP clients.

3. Validations​

Basic validations are performed during email processing:

  • Email size is checked against MAX_EMAIL_SIZE_BYTES. Oversized messages are rejected with SMTP error 552.
  • Required fields (sender, recipients, message body) are verified before the message is accepted for further processing.

4. Message Processing​

  • Incoming messages are parsed and queued into an internal tokio::sync::mpsc channel.
  • A dedicated worker task consumes messages from this channel and publishes them to the configured AMQP exchange.
  • This design provides backpressure handling, ensuring the server remains responsive even under load.

5. Storage​

  • By default, LSMTPD is stateless and processes messages in memory for maximum speed.
  • If the AMQP broker is unavailable, messages are written as JSON files to:
/tmp/lsmtp/<uuid-v4>.json

This ensures no email is lost and allows manual recovery later.