rusmes-smtp

SMTP protocol implementation for RusMES, targeting RFC 5321 compliance with modern extensions.

Architecture

SmtpServer
  |-- accepts TCP connections
  |-- spawns SmtpSession per connection
  |     |-- SmtpState machine
  |     |-- Command parser (nom-based)
  |     |-- Response builder
  |     |-- Authentication (via rusmes-auth)
  |     '-- Hands off Mail to MailProcessorRouter
  '-- TLS support via rustls

Modules

Module	Description
`server`	`SmtpServer` - TCP listener, connection dispatch
`session`	`SmtpSession` - per-connection state machine and command handling
`session/data`	DATA tempfile spill — payloads above threshold (default 1 MiB) spill to `tempfile::NamedTempFile`, returning `MessageBody::Large`; configurable via `SmtpConfig::data_tempfile_threshold`
`outbound_pool`	Outbound connection pool — reusable TLS+TCP sessions to relay destinations
`command`	`SmtpCommand` enum - all SMTP commands with parsed parameters
`parser`	`parse_command()` - nom-based SMTP command parser
`response`	`SmtpResponse` - SMTP reply codes and messages

SMTP Commands

Command	Status	Description
EHLO/HELO	Implemented	Client greeting with capability negotiation
MAIL FROM	Implemented	Set envelope sender
RCPT TO	Implemented	Add envelope recipient
DATA	Implemented	Message body transfer
RSET	Implemented	Reset transaction
QUIT	Implemented	Close connection
STARTTLS	Implemented	TLS upgrade
AUTH	Implemented	PLAIN, LOGIN, CRAM-MD5, SCRAM-SHA-256 mechanisms
mTLS	Implemented	Mutual TLS — client certificate verification on TLS listeners
NOOP	Implemented	No operation
VRFY	Stub	Verify address

Session States

Initial -> Connected (after greeting)
  -> Authenticated (after AUTH or if auth not required)
    -> MailTransaction (after MAIL FROM)
      -> Data (after DATA, collecting message body)
    -> Quit

Configuration

pub struct SmtpConfig {
    pub hostname: String,         // Server hostname for greeting
    pub max_message_size: usize,  // Max message size in bytes
    pub require_auth: bool,       // Require AUTH before relay
    pub enable_starttls: bool,    // Advertise STARTTLS
}

Dependencies

rusmes-proto - mail types
rusmes-core - mail processing router
rusmes-auth - authentication backend
nom - command parsing
rustls / tokio-rustls - TLS
tokio - async networking

Tests

cargo test -p rusmes-smtp   # 165 tests

Tests cover: command parsing, response formatting, session state transitions, EHLO capability negotiation, and mail address extraction.

rusmes-smtp 0.1.2