Crate mailrs_smtp_client 

Outbound SMTP client primitives: MX resolution, DANE/STARTTLS, response parsing.

mailrs-smtp-client is the send-side counterpart to mailrs-smtp-proto. It is not a full mail user agent — it is the wire-level pieces an MTA needs to deliver a message that has already been built: looking up MX records, choosing the right relay, opening a TLS connection that can be verified against DNSSEC-anchored TLSA records (RFC 7672 DANE), and reading SMTP replies that wrap across multiple lines.

Built on tokio + rustls + hickory_resolver. Extracted from mailrs, a Rust mail server, and published independently so anyone writing an MTA, a delivery-test harness, or a bounce probe in Rust can share the same battle-tested foundation.

Quick start

use mailrs_smtp_client::{MxCache, SmtpConnection, TokioResolver, sort_mx_records};
use std::time::Duration;

let resolver = TokioResolver::builder_tokio()?.build()?;
let cache = MxCache::new(Duration::from_secs(300));

let mut records = cache.resolve(&resolver, "example.com").await?;
sort_mx_records(&mut records);
let primary = records.first().ok_or("no MX")?;

// connect (reads the banner internally), EHLO, upgrade to TLS, then send
let mut conn = SmtpConnection::connect(&primary.exchange, 25).await?;
conn.ehlo("client.example.org").await?;
let mut conn = conn.starttls(&primary.exchange).await?;
conn.ehlo("client.example.org").await?;
conn.deliver(
    "sender@example.org",
    &["bob@example.com"],
    b"Subject: hi\r\n\r\nhello\r\n",
).await?;
conn.quit().await?;

Module overview

• mx — DNS MX lookup with preference-sort, in-memory cache, and A-record fallback for domains without an MX.
• dane — TLSA record resolution and certificate verification (RFC 7672) to defend STARTTLS against active MITM.
• connection — SmtpConnection wraps the TLS-upgradable read/write loop with per-command timeouts (TimeoutConfig).
• response — single- and multi-line reply parser (RFC 5321 §4.2.1).

What this crate does NOT do

• No DKIM signing, no SPF, no DMARC. Use mail-auth upstream.
• No queue / retry / DSN. See mailrs-outbound-queue.
• No SMTP server. See mailrs-smtp-proto for the receive-side state machine.

Re-exports

pub use connection::SmtpConnection;

pub use connection::TimeoutConfig;

pub use dane::DaneVerifier;

pub use dane::TlsaRecord;

pub use dane::dane_tls_config;

pub use dane::resolve_tlsa;

pub use mx::MxCache;

pub use mx::MxRecord;

pub use mx::fallback_to_domain;

pub use mx::resolve_mx;

pub use mx::sort_mx_records;

pub use response::SmtpResponse;

pub use response::parse_response;

pub use tls_outcome::StarttlsResult;

pub use tls_outcome::TlsOutcome;

Modules

connection

SmtpConnection — async TLS / STARTTLS connection state machine + timeouts.

dane

RFC 7672 DANE (DNS-Based Authentication of Named Entities): TLSA record lookup + TLS verification config.

mx

MX record lookup + priority sorting + fallback_to_domain for MX-less destinations.

response

RFC 5321 multi-line response parser (250-XXX continuation lines). Parsed SMTP reply from the wire — see parse_response.

tls_outcome

Structured outcomes of STARTTLS attempts — discriminated enum suitable for direct mapping to mailrs_tls_rpt::FailureType (RFC 8460 §4.3) and any downstream decision that wants more than an opaque io::Error. Structured TLS outcomes for STARTTLS attempts.

Type Aliases

TokioResolver

A Resolver used with Tokio