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Workflow

This page explains the lifecycle of an email inside LSMTPD — from the moment a client connects, to when the message is delivered into AMQP.

LSMTPD workflow

1. Client Connection​

  • A client (e.g., another mail server) connects to LSMTPD over SMTP.
  • LSMTPD spawns a new async task (tokio::spawn) to handle this connection.
  • A 180-second timeout is applied to prevent idle sessions.

2. SMTP Handshake​

The client and server exchange initial commands:

  • EHLO / HELO → client introduces itself.
  • Server responds with its configured SERVER_NAME.

3. Envelope Setup​

The client defines sender and recipients:

  • MAIL FROM:<sender> → sets the envelope sender.
  • RCPT TO:<recipient> → adds one or more recipients.

4. Message Transfer​

  • The client issues the DATA command.
  • The server accepts the raw email body until the terminating . (dot on a new line).
  • During this stage, LSMTPD enforces:
    • Size limits (reject with 552 if exceeded).
    • Basic SMTP formatting.

5. Validation​

Before final acceptance:

  • Required fields (MAIL FROM, at least one RCPT TO, and message body) must be present.
  • If validation fails, LSMTPD responds with the appropriate error code and discards the message.

6. Message Queueing​

If valid:

  • The message is placed into a tokio::mpsc channel.
  • A dedicated worker task reads from this channel and prepares the message for AMQP.

7. AMQP Delivery​

  • The worker publishes the email into the configured AMQP exchange with the specified routing_key.
  • If AMQP is available, delivery is immediate.
  • If AMQP is unreachable, the message is temporarily written to:
/tmp/lsmtp/<uuid-v4>.json

This ensures no messages are lost.

8. Connection Close​

  • Once processing is complete, the client may send QUIT to end the session.
  • LSMTPD closes the connection and frees resources.