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Swift email SDK

The Swift email SDK for macOS, iOS and Linux.

Receive email in Swift without running a mail server. Create an address, await the next message, read its text, HTML and attachments, and reply in the thread. Mail is stored encrypted and opened in your own process, with async and await from end to end.

Install the Swift package
.package(url: "https://mailcycle.email/swift/mailcycle.git", from: "0.1.0")
Create a free accountRead the Swift guide

Mailcycle on Swift Package Manager · Swift 5.9 or later · Apache 2.0 · Free tier, no card

Quickstart

Receive and send email in Swift.

Sign in, create an address, wait for a message and answer it. That is the whole loop, and it is the same loop whether the mail is a sign-up code, a customer or a receipt.

  1. 1Install Mailcycle and set MAILCYCLE_PHRASE to your account's recovery phrase.
  2. 2Create an address with mc.addresses.create(). It receives mail straight away.
  3. 3Wait for the next message with mc.messages.waitFor(), or take it from a webhook.
  4. 4Reply with mc.messages.send(), threaded under the original.
The full Swift guide
Swift: create an address, wait for mail, reply
import Mailcycle

let environment = ProcessInfo.processInfo.environment
let mc = try await Mailcycle.signIn(phrase: environment["MAILCYCLE_PHRASE"]!)

// A new address that receives mail, sealed to your key.
let address = try await mc.addresses.create(NewAddress(prefix: "signup", label: "Test run 42"))
print(address.emailAddress) // signup-x7k2@...

// Wait for the verification email, decrypted here.
let message = try await mc.messages.waitFor(WaitFor(addressId: address.id, subject: "verification"))
print(message.subject, message.text)

// Reply in the same thread.
_ = try await mc.messages.send(Outgoing(
    from: address.emailAddress,
    to: message.replyTo,
    subject: "Re: \(message.subject)",
    text: "Thanks, confirmed.",
    replyTo: message
))
What it does

The whole email API, from Swift.

Everything the Mailcycle console can do is in the package: addresses, inbound mail, sending, webhooks, events, API keys and devices.

Create email addresses from Swift

mc.addresses.create() mints a new address on a shared domain, or on your own domain from Scale, and it receives mail at once. Give it a prefix and a label sealed on your machine.

Receive email and wait for it

mc.messages.waitFor() listens on the event stream and returns the next matching message, filtered by sender or subject, opened and ready to read. Without a stream it checks every five seconds.

Read and parse incoming email

mc.messages.all() and .attachment() return the sender, subject, plain text, the original HTML, a safe HTML with remote images and trackers removed, and every attachment as bytes.

Send email and reply in the thread

mc.messages.send() sends from an address your account owns, with up to ten files. Pass the message you are answering and the threading headers are filled in.

Inbound email webhooks

Register a URL and every message.received event is POSTed to it, signed with HMAC-SHA256 and retried. verifyWebhookSignature() checks the signature in one line.

A live event stream

mc.events.stream() carries the same events over a WebSocket the moment they happen, and reconnects after a drop. Events carry ids, never content.

Email testing

Test email verification in XCTest, Swift Testing and XCUITest.

Create an address in a test's setUp, drive your sign-up screen with XCUITest, then await waitFor and read the code from message.text. A timeout throws MailcycleError.api with the code timeout, so the test fails with a reason.

  • A real address on a real domain, so the mail goes through the same path your users' does.
  • One call blocks until the message lands, filtered by subject or sender, with a timeout you set.
  • Text, HTML and attachments come back parsed, so pulling out a code or a link is a regular expression.
  • Mail received from the moment you start waiting counts, even if it lands before the stream is up.
Inbound email webhooks

Handle incoming email in Vapor and Hummingbird.

Rather than polling, let the mail come to you. A webhook POSTs each event to your URL the moment it happens, signed so you can prove it came from Mailcycle, and retried if your server is down.

Events carry ids, never content. Fetch the message by id and it is opened in your code, exactly as it would be otherwise. Webhooks are on Operator and above.

Webhooks in the docs
Vapor: verify a webhook before trusting it
import Mailcycle
import Vapor

app.post("hooks", "mailcycle") { req async throws -> HTTPStatus in
    // The body exactly as it arrived, before any parsing.
    let raw = req.body.data.map { [UInt8]($0.readableBytesView) } ?? []
    let signature = req.headers.first(name: "Mailcycle-Signature")
    guard verifyWebhookSignature(secret: secret, rawBody: raw, header: signature) else {
        return .unauthorized
    }

    struct Event: Decodable { let type: String; let payload: [String: String] }
    let event = try JSONDecoder().decode(Event.self, from: Data(raw))
    if event.type == "message.received", let id = event.payload["messageId"] {
        let message = try await mc.messages.get(id)
        try await handle(message.subject, message.text)
    }
    return .noContent
}
No mail server

Instead of an SMTP library and a mailbox.

Foundation has no SMTP or IMAP, and MFMailComposeViewController only opens a compose sheet for a person to send. Doing mail in code means a SwiftNIO-based SMTP or IMAP library, a mailbox and its password, and a MIME parser.

Here an address is one awaited call, mail arrives on a webhook or through waitFor, and each message comes back as a parsed struct: sender, subject, text, safeHTML with remote content removed, and attachments as Data.

It is not a bulk sender. If you need newsletters or high volumes of transactional mail, use a sending service for that; Mailcycle is for addresses your code owns and the mail that comes back to them.

End-to-end encrypted

Mail is opened in your Swift code, not on our servers.

Each address has a key pair derived from your recovery phrase. Incoming mail is sealed to its public key before anything is written down, and the package derives the private key on your machine to open it.

How the encryption works
  • Signing in proves you hold the key by answering a challenge, so the phrase never crosses the wire.
  • Bodies, attachments, address labels and API key names are all sealed. The server stores ciphertext.
  • Mail sealed by one SDK opens in every other, and in the app, because they share one published format.
  • The package is checked against Mailcycle's published test vectors, including the cases that must be refused.
  • Because the server cannot read mail, there is no server-side search. Search runs where the mail is opened.
Use cases

What developers build with the Swift SDK.

It runs in command-line tools, server-side Swift, macOS utilities and test suites: anywhere Swift code needs an email address of its own.

End-to-end tests for sign-up and login

Verification emails, one-time passcodes, magic links, password resets and invitations, checked by a real test in XCTest, Swift Testing and XCUITest.

A support or orders inbox your code answers

Give an address to a queue, read what customers send, and reply from the address they wrote to, threaded under their message.

Receipts, invoices and alerts as data

Point suppliers, monitoring and other services at an address and pull the text and attachments straight into your own records.

An address per customer, job or device

Mint addresses on demand and keep each stream of mail separate, with its own label, and delete one when the job is done.

Building an AI agent? It can have an inbox too, through the Mailcycle MCP server, without writing any Swift at all.

The package

Swift SDK at a glance.

Maintained by Mailcycle, the team that runs the API.

The Mailcycle Swift SDK at a glance
PackageMailcycle
RegistrySwift Package Manager
Install withSwift Package Manager, in Package.swift
NeedsSwift 5.9 or later
AlsoOn macOS 13, iOS 16 or Linux. Every call uses async and await.
LicenceApache 2.0, free to use in your own applications
APIJSON over HTTPS at api.mailcycle.email
GuideThe Swift guide in the docs

What each plan unlocks. Free receives mail. A signed-in session sends from Personal up. API keys and webhooks start at Operator, and sending from an API key at Scale. See pricing.

Questions

Swift email SDK questions.

Add the package with Swift Package Manager, sign in with try await Mailcycle.signIn(phrase:), and create an address with mc.addresses.create(). Mail sent to it is stored encrypted, and mc.messages.all() or mc.messages.waitFor() returns it decrypted and parsed. No mail server and no IMAP.

Swift 5.9 or later, on macOS 13, iOS 16 or Linux. It suits command-line tools, server-side Swift and test suites. Do not ship a recovery phrase or an API key inside an app you distribute: either one gives access to the account.

Swift Package Manager takes any Git URL, and the package is published at https://mailcycle.email/swift/mailcycle.git. Add that URL in Package.swift or in Xcode's Add Package Dependencies, and versions resolve from its tags as usual.

The package is free and open source under the Apache 2.0 licence. The Mailcycle Free plan costs nothing and needs no card: 3 email addresses and 3,000 messages received a month, receive only. Sending starts on Personal at $7 a month, API keys and webhooks on Operator at $15, and sending from an API key on Scale at $19.

No. Incoming mail is sealed to the address's public key before it is stored, and the private key is derived from your recovery phrase inside your Swift process. The server holds ciphertext it cannot open. Mail you send leaves in plain text so the recipient's server can read it, and the copy kept in Sent is sealed like the rest.

Call mc.addresses.create() at the start of the run and delete the address at the end, which frees its place on your plan. Each address receives mail as soon as it exists. This is for testing your own product and for mail meant for you: registering accounts on other services in bulk, or against their terms, breaks the Acceptable Use Policy.

No. Mailcycle is inbound email infrastructure, and sending is there to answer what arrives. Sending is capped at 60 messages an hour per account and 20 an hour per device or agent, with a daily limit set by the plan, and the Acceptable Use Policy rules out bulk, marketing and outreach mail.

The API is plain JSON over HTTPS at api.mailcycle.email, so anything can call it. Message content comes back encrypted, though, and the package is what derives the keys and opens it. Without an SDK you would implement the published encryption format yourself.
Other languages

The same API in every language you use.

Nine SDKs share one encryption format, so a message created by one opens in all the others.

Give your Swift code an email address.

Create an account, create an address, and read what arrives. The free tier needs no card.

Get startedRead the Swift guide