restate-email-endpoint

Standalone endpoint hosting the email service for Restate.

Install

cargo install restate-email-endpoint

Configuration

The restate-email binary reads a JSON, YAML, or TOML configuration file and applies RESTATE_EMAIL_ environment overrides. At least one transport must be configured.

identity_keys = ["publickeyv1_w7YHemBctH5Ck2nQRQ47iBBqhNHy4FV7t2Usbye2A6f"]

[transports.transactional]
provider = "resend"
api_key = "re_..."

[attachments]
max_attachment_bytes = 26214400
max_total_bytes = 41943040

[attachments.resolvers.docs]
type = "opendal"
service = "s3"
bucket = "example-docs"
root = "/outbound"

restate-email --config restate-email.toml --port 9080

The Restate service name is Email. Invoke its send handler with a restate_email::SendRequest whose transport key matches a configured entry. In the example above, an attachment reference such as docs:reports/quarterly.pdf is resolved as reports/quarterly.pdf within the configured S3 bucket and root before Resend receives the message.

The [attachments] section is optional. When present, every configured transport is wrapped with attachment preparation. The resolver key is the reference routing prefix, and all fields other than type and service are passed to the selected OpenDAL service as operator options. Supported services include azblob, fs, gcs, http, and s3.

With the transport-lettre feature (also enabled by transport-all), SMTP transports are configured through a Lettre connection URL:

[transports.notifications]
provider = "smtp"
url = "smtps://user:password@smtp.example.com:465"

The URL carries credentials, host, port, EHLO name, and TLS mode (smtp://, smtps://, and the ?tls= query parameter) as documented by Lettre's AsyncSmtpTransport::from_url.

Without [attachments], byte-backed messages work unchanged and provider transports reject unresolved references terminally. Missing objects, unsupported references, access failures, and size violations are terminal; transient storage failures are retryable. Resolution occurs during the existing send_email Restate action, and resolved bytes are not journaled. Use immutable or versioned references when retries must resolve identical content.

Request Identity

Restate signs every request it makes to a service endpoint when the runtime is configured with a request identity key. identity_keys lists the matching publickeyv1_... public keys; with at least one key configured the endpoint rejects unsigned requests. Multiple keys stay valid at once, so rotation is a config change: add the new key, switch the runtime to the new private key, then drop the old one. The environment override accepts a comma-separated list:

RESTATE_EMAIL_IDENTITY_KEYS="publickeyv1_old,publickeyv1_new" restate-email --config restate-email.toml

Without identity_keys the endpoint accepts unsigned requests. Identity keys authenticate the Restate runtime to this endpoint; callers authenticate to Restate ingress separately (see email-transport-restate).

Feature Flags

The default build is batteries-included: minimal plus transport-all. For a slimmer binary, disable default features and pick minimal plus the transports you need.

• minimal: the component defaults, queue-payload schemas, RFC 5322 string-address compatibility, and tracing. Every build wants this.
• attachment-opendal: enables endpoint attachment preparation and the OpenDAL services listed above. It is enabled by transport-all and transport-all-wasm. Without it, [attachments] limits still apply to byte-backed attachments, but resolvers cannot be configured.
• transport-lettre: enables the SMTP transport and its provider = "smtp" endpoint configuration. It is enabled by transport-all.
• transport-resend: enables the Resend transport and provider-option deserialization. It is enabled by transport-all and transport-all-wasm.
• transport-all: enables attachment preparation and every transport exposed by email-kit.
• transport-all-wasm: enables attachment preparation and every email-kit transport that supports wasm32-unknown-unknown.

The binary target requires at least one transport feature and fails the build with a clear error without one:

cargo install restate-email-endpoint --no-default-features --features minimal,transport-lettre

See the generated feature graph for activation details.

License

Licensed under either of

• Apache License, Version 2.0 (LICENSE-APACHE or https://www.apache.org/licenses/LICENSE-2.0)
• MIT license (LICENSE-MIT or https://opensource.org/licenses/MIT)

at your option.

Contribution

Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in the work by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.