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Sending email

Welcome messages, password resets, receipts, alerts — most apps send transactional email. Rustango gives you a Mailer trait with swappable backends (console for dev, SMTP for production, an in-memory recorder for tests), a fluent Email builder with header-injection protection, and template rendering. Write mailer.send(&email) once; switch from printing to your terminal to real SMTP with a one-line change — like Django's email framework.

Email in Rustango: an Email builder (to/subject/body/html) is validated against header injection, then sent through the Mailer trait — ConsoleMailer in dev, SmtpMailer in prod, InMemoryMailer in tests

New to a term here? transactional email, SMTP, mailer backend — see the glossary.

Source: rustango::email (Mailer, Email, ConsoleMailer, InMemoryMailer, NullMailer, SmtpMailer, BoxedMailer, send_mail, MailError) — behind the email feature (on by default). SMTP needs the email-smtp feature.

Runnable version: every snippet is copied from email_doc.rs (cargo test -p rustango --test email_doc); the send helpers and attachments are dogfooded by email_send_helpers.rs and email_attachments.rs.

Table of contents

Step 1 — Build an email

Email is a fluent builder. Set recipients, subject, and a text and/or HTML body:

use rustango::email::Email;

let email = Email::new()
    .to("[email protected]")
    .from("[email protected]")
    .subject("Welcome")
    .body("Thanks for signing up.")              // plain-text part
    .html_body("<p>Thanks for signing up.</p>"); // optional HTML part

.cc(...), .reply_to(...), and attachments are available too.

Step 2 — Pick a mailer

Every backend implements Mailer, so your code never names the concrete type — hold a BoxedMailer (Arc<dyn Mailer>):

BackendFeatureUse for
ConsoleMaileremaildev — prints the message to stdout
SmtpMaileremail-smtpproduction — real delivery over SMTP
InMemoryMaileremailtests — records messages, sends nothing
FileMaileremaildev/CI — writes each message to a file
NullMaileremaildisable email entirely

Build it from config so it differs per environment (ConsoleMailer locally, SmtpMailer in prod) via email::from_settings(&settings.email).

Step 3 — Send it

Email::send takes any &dyn Mailer:

email.send(&mailer).await?;

For a quick one-off, send_mail skips the builder:

use rustango::email::send_mail;

send_mail(
    &mailer,
    "Your report is ready",                  // subject
    "Download it from your dashboard.",      // body
    Some("[email protected]"),             // from (or None for the default)
    &["[email protected]", "[email protected]"],  // recipients
).await?;

send_many sends a batch in one call.

Validation and header-injection safety

Email::validate() runs before sending (and you can call it yourself). It rejects incomplete messages and defends against header injection — a newline smuggled into a header is how attackers add a hidden Bcc:

// Missing recipients or an empty subject → MailError::InvalidMessage
Email::new().subject("hi").validate()?;          // Err: no recipients

// A CRLF in any header field → MailError::BadHeader (Django's BadHeaderError)
Email::new()
    .to("[email protected]")
    .subject("Hello\r\nBcc: [email protected]")  // injection attempt
    .body("x")
    .validate()?;                                  // Err: BadHeader

Both are verified in the backing test.

Testing email

Use InMemoryMailer — it records every message instead of sending, so tests assert on what would have gone out, with no network:

use rustango::email::InMemoryMailer;

let mailer = InMemoryMailer::new();
welcome_flow(&mailer).await?;          // your code under test

let sent = mailer.sent();              // Vec<Email>
assert_eq!(sent.len(), 1);
assert_eq!(sent[0].to, vec!["[email protected]".to_string()]);
assert_eq!(sent[0].subject, "Welcome");

Templates

For anything beyond a line of text, render the body from a Tera template instead of inlining HTML. The email_templates feature's EmailRenderer follows a name.subject.txt / name.txt / name.html convention — one template set produces the subject, the plain-text part, and the HTML part together, so the three never drift. The Mailable trait packages "a thing that knows how to turn itself into an Email" for reusable messages.

Send it off the request

Sending email inline makes the user wait on your SMTP server and couples the response to its availability. Send it from a background job instead — the handler returns immediately and a worker delivers it (with retries if SMTP is down):

// in the handler: enqueue, don't send inline
queue.dispatch(&SendWelcomeEmail { user_id }).await?;

// the job (see the Background jobs guide):
async fn run(&self) -> Result<(), JobError> {
    let email = Email::new().to(/* ... */).subject("Welcome").body("...");
    email.send(&*mailer).await.map_err(|e| JobError::Retryable(e.to_string()))?;
    Ok(())
}

The email_jobs feature wires this up for you.

Reference

Email builder: to · cc · from · reply_to · subject · body · html_body · attachments · validate() · send(&mailer).

Helpers: send_mail(mailer, subject, body, from, &recipients) · send_many(mailer, &emails) · from_settings(&EmailSettings).

MailError: InvalidMessage (incomplete) · BadHeader (CRLF injection) · Transport (backend/delivery failure).

See also

  • Background jobs — deliver email off the request with retries.
  • Account flows — password-reset / verification / magic-link emails built on this.
  • HTML views — the Tera engine email templates also use.
  • Caching — the same swap-the-backend trait pattern.
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