Rust SDK

A Rust client for the API that does the encryption the Mailcycle apps do, on your machine. With it a program can create addresses that receive mail, open what arrives, and send. The recovery phrase never leaves the machine it runs on.

It needs Rust 1.75 or later.

[dependencies]
mailcycle = { version = "0.5", features = ["events"] }

The events feature adds the live stream; leave it off and the WebSocket dependency is not built.

Sign in

use mailcycle::Mailcycle;

let mc = Mailcycle::sign_in(&std::env::var("MAILCYCLE_PHRASE")?, Default::default())?;

This is the same handshake the apps use: nothing secret crosses the wire, and it works on every plan.

With an API key instead, on Operator and up:

let mc = Mailcycle::with_api_key(
    &std::env::var("MAILCYCLE_API_KEY")?,
    std::env::var("MAILCYCLE_PHRASE").ok().as_deref(),   // None to manage without opening mail
    Default::default(),
)?;

Without the phrase a client can manage addresses, devices and metadata, but cannot open mail or create an address that receives it, because the address’s key is derived from the phrase. With both, the phrase is checked against the key’s account before anything is opened.

Each sign_in opens a 30-day session. Keep mc.session_token() somewhere private and pick it up with Mailcycle::resume(token, phrase, options) rather than signing in on every run.

Addresses

use mailcycle::NewAddress;

let address = mc.addresses().create(NewAddress {
    prefix: Some("signup".into()),
    label: Some("Test run 42".into()),
    ..Default::default()
})?;

let all = mc.addresses().list()?;

The public half of the address’s key goes up with the request so the mail server can seal what arrives; the private half stays on your machine. The label is sealed locally, and the server stores it as ciphertext.

With no prefix or local_part, the server rolls a name like quiet_falcon. Set style to choose the kind:

use mailcycle::NameStyle;

let address = mc.addresses().create(NewAddress {
    style: Some(NameStyle::Person),   // maya.holt
    ..Default::default()
})?;

The styles are Person, Business, Team and Neutral, the default. style is ignored when prefix or local_part is set.

To pick a name before creating, roll a few:

let names = mc.addresses().roll_names(NameStyle::Business, None)?;   // up to 8, like harbor-supply42

let address = mc.addresses().create(NewAddress {
    local_part: Some(names[0].clone()),
    ..Default::default()
})?;

A rolled name works as local_part on every plan; any other name needs Scale and up. The names are free when rolled, but someone can take one before you create it, and then create is refused. Pass a domain to roll for it; None uses the one create would.

Reading mail

let messages = mc.messages().all(&address.id, None)?;
for message in &messages {
    println!("{} {}", message.from_address, message.subject);
}

let data = mc.messages().attachment(&messages[0], 0)?;   // Vec<u8>, opened locally

A message that will not open, because there is no phrase or it is the wrong one, comes back with opened: false and empty content rather than an error, so one unreadable message does not hide the rest of a page.

message.html is the HTML as the sender wrote it. message.safe_html has every remote image, stylesheet and embed removed, as the apps do it, and is the one to render; message.blocked lists what was taken out.

Waiting for a message

use mailcycle::WaitFor;

let code = mc.messages().wait_for(WaitFor {
    address_id: Some(&address.id),
    subject: Some("verification"),
    ..Default::default()
})?;

Listens on the event stream where it is available and polls the address otherwise, and returns the message opened. If nothing matching arrives in time it returns Error::Api with code = "timeout". Mail received from since on counts, even if it landed before the stream was up; it defaults to now, so to wait for a reply to something you are about to send, take the time before sending.

Sending

use mailcycle::Outgoing;

mc.messages().send(Outgoing::new(
    &address.email_address,
    "someone@example.com",
    "Hello",
    "Hi",
))?;

Set reply_to to thread a reply. A session can send from Personal up; an API key on Scale and up. Free receives only.

send returns a SendResult: message_id is the Message-ID header, and sent_copy_id is the id of the copy in Sent, which the other message calls take, or None when the address keeps none.

Attachments take raw bytes and the client encodes them. A message can carry up to ten files, 3.5 MB between them. Give a file a content_id to show it inline, where the HTML says cid: and that id. Program and script files, like .exe and .js, are refused.

use mailcycle::OutgoingAttachment;

let pdf = std::fs::read("invoice.pdf")?;
let logo = std::fs::read("logo.png")?;
let files = [
    OutgoingAttachment { mime_type: Some("application/pdf"), ..OutgoingAttachment::new("invoice.pdf", &pdf) },
    OutgoingAttachment {
        mime_type: Some("image/png"),
        content_id: Some("logo"),
        ..OutgoingAttachment::new("logo.png", &logo)
    },
];
mc.messages().send(Outgoing {
    html: Some(r#"<img src="cid:logo"><p>Attached.</p>"#),
    attachments: &files,
    ..Outgoing::new(&address.email_address, "someone@example.com", "Your invoice", "Attached.")
})?;

Events and webhooks

let stream = mc.events().stream()?;
while let Some(event) = stream.next_event(None) {
    println!("{}", event.event_type);
}

Events carry ids, never content, and are not replayed after a drop, so Hooks::on_reconnect, passed to stream_with, is where to refetch what you are showing.

To check a webhook came from Mailcycle, pass the body exactly as it arrived:

use mailcycle::verify_webhook_signature_now;

verify_webhook_signature_now(secret, raw_body, header);

API keys

let created = mc.api_keys().create("Deploy script")?;
println!("{}", created.token);   // shown once; keep it somewhere private

for key in mc.api_keys().list()? {
    println!("{} {:?} ...{}", key.id, key.name, key.hint);
}
mc.api_keys().revoke(&created.api_key.id)?;

Listing, creating and revoking keys need a signed-in session, so a leaked key cannot make more of itself. The name is sealed on your machine, and the server stores it as ciphertext. A name that will not open comes back as None.

sign_out() on a client built with an API key does nothing. A key is not a session; it works until it is revoked.

Webhooks

use mailcycle::WebhookUpdate;

let created = mc.webhooks().create("https://example.com/hooks/mail", &["message.received"], "Orders")?;
let secret = created.secret;   // shown once, for checking signatures

mc.webhooks().update(&created.webhook.id, WebhookUpdate { enabled: Some(false), ..Default::default() })?;
mc.webhooks().test(&created.webhook.id)?;

let page = mc.webhooks().deliveries(&created.webhook.id, None, None)?;
for delivery in &page.deliveries {
    println!("{} {} {:?}", delivery.event_type, delivery.status, delivery.response_status);
}

Webhooks are on Operator and up. list() returns the webhooks with their names opened, and every event type one can ask for. rotate_secret returns a new secret once, and redeliver sends a logged event again with the same event id. Adding, changing, rotating and removing need a signed-in session; an API key can list, test, read deliveries and redeliver.

Sessions

for session in mc.sessions().list()? {
    println!("{} {} {}", session.id, session.expires_at, session.current);
}
mc.sessions().revoke_others()?;

revoke(id) ends one session. All three need a signed-in session.

Deleting the account

mc.delete_account()?;

Erases the account and everything in it at once, with no grace period. It takes &mut self, as sign_out does, so the client must be declared mut. Its addresses are retired and never issued again. It needs a signed-in session.

mc.subscription() returns the plan, renewal date and balance, and mc.plans() lists every plan.

Pairing devices

// The text of the device's QR code, or the eight digits it shows.
let device = mc.devices().pair("MC1:12345678:...", "Kitchen Pi")?;

mc.devices().assign(&device.id, &address.id)?;     // hands the keys over too
mc.devices().unassign(&device.id, &address.id)?;
mc.devices().hand_over_keys(&device.id)?;          // again, on its own

This is what the app’s Add device screen does, done by a script. It needs the phrase and a signed-in session. The name is sealed on your machine.

Each device has a wrap secret, and its address keys are sealed under it. The account keeps a copy in the device’s sealed profile, so keys can be handed over again when an address is assigned or taken off.

There are two ways the secret gets here. The QR code carries the device’s own secret, so it never passes through Mailcycle. The eight digits carry no secret, so the client makes one and seals it to a key the device published. The server relays that key, so it could put its own in its place at pairing and read the keys that follow. The QR path does not have that gap. Use it where you can.

A device on an older build publishes no key. Pairing it by its digits fails with an error that says to pair it in the app, or pass its QR code instead.

Errors

Everything returns Result<T, mailcycle::Error>. An API failure is Error::Api carrying the HTTP status and the API’s own code, which is what to match on.

How the encryption is checked

It is checked against the published test vectors, including the refusals.