zeno-channel-email

Email channel for Zeno backed by Resend. Hosts a Starlette + uvicorn server for Resend's inbound webhook and posts outbound replies (with correct RFC 5322 threading headers) to Resend's REST API.

Verifies inbound webhooks with Svix-style HMAC-SHA256 signatures and enforces a sender allowlist. Replies to an inbound email land in the same thread in Gmail, Outlook, and Apple Mail because the channel owns the Message-ID / In-Reply-To / References header math.

Install

uv add 'zeno-framework[email]'
# or, without the AI package:
uv add zeno-channel-email

Minimal usage

from zeno.channels.email import EmailChannel

channel = EmailChannel(
    api_key="re_...",                                    # Resend API key
    signing_secret="whsec_...",                          # Resend webhook secret
    from_address="zeno@mail.example.com",
    allowed_senders=("you@example.com",),
    host="127.0.0.1",
    port=8080,
    webhook_path="/webhook",
)
# Then pass it to ZenoApp(channels=[channel]) exactly like any Channel.

The signing secret rotation flow uses Svix's multi-secret window: add the new secret in the Resend dashboard first, restart the process with the new env var, then remove the old secret. No downtime.

See apps/zeno-example-chat for a runnable reference wiring that turns the email channel on only when the relevant env vars are set.

Behavior

• Signature verification: Resend signs inbound webhooks with Svix. Bad or missing signature → 401. Every other drop (stale timestamp, unknown event type, unallowlisted sender, malformed JSON, fetch_received failure, queue full) returns 200 so Resend doesn't retry and so the 401 audit signal stays meaningful.
• Two-phase inbound: Resend's email.received webhook carries metadata only. The channel GETs /emails/received/{email_id} for the body and headers before enqueuing an IncomingMessage.
• Inbound → IncomingMessage: user_id = normalized lower-case From: address (parsed with email.utils.parseaddr to defeat display-name injection), text = preferred text/plain body with HTML fallback, thread_key = same as user_id so replies route back. RFC-5322 headers are cached per-thread in-process so outbound replies can build correct In-Reply-To / References. Attachments on inbound are ignored in 0.7.0.
• Outbound: text + optional attachments via POST /emails. multipart/alternative is synthesized from the agent's plaintext reply (no user-authored HTML templates). Message-ID is generated per reply. References chain is capped at 50 entries to defend against crafted-header DoS.
• Capabilities: supports_threading=True, supports_images=False (attachments flow outbound only in 0.7.0).

Security notes

• signing_secret and api_key are marked repr=False on the channel dataclass so they don't leak in traceback / debug output.
• The 300 s Svix timestamp window is mandatory (replay defense).
• allowed_senders is normalized to lower-case at construction; inbound From: is also normalized so the allowlist check is case-insensitive.
• Resend's POST /emails caps a single email at 40 MB (attachments counted after base64 encoding).

Testing

uv run pytest packages/zeno-channel-email

Tests use httpx.MockTransport for Resend's REST surface and the Starlette TestClient for the router. No Resend account required in CI.

See also

• zeno-channel-cli, zeno-channel-sendblue — other first-party channels.
• docs/adapters.md — writing your own Channel.

Part of the Zeno framework.