feat(email): split email-send from the full email provider

[senamakel]

Splits the email feature into a send-only half and the full provider, so a host that only ever delivers mail stops linking the IMAP receive stack.

email-send = ["dep:lettre"]
email      = ["email-send", "dep:async-imap", "dep:mail-parser"]

Why

This is a prerequisite for moving OpenHuman's channels onto the TinyChannels bus module, and without it that move sheds almost nothing.

OpenHuman's voice feature requires tinychannels/email for one reason: voice/audio_toolkit/ops.rs emails a generated podcast as an attachment. It uses exactly EmailChannel::new(cfg) and send_message(msg) — inherent methods, never the Channel trait, never a mailbox. But email was one feature, so a voice-enabled build linked the whole IMAP receive stack it could never call.

Measured on OpenHuman's product profile (dep-sim.py, cross-checked against an independent reachability walk over cargo tree --prefix depth — both agree):

Cut	delta
async-imap + mail-parser (receive)	−15 pkg / −13 names
prost (lark)	−4 pkg / −4 names
tinychannels root edge entirely	−20 pkg / −18 names

So ~17 of the 18 names attributable to this crate are the two provider features, not the crate itself. Without this split, a voice-enabled host keeps async-imap + mail-parser no matter where the channel providers run, and the bus move sheds only the 4 lark names.

At this crate's own level the split is 174 → 156 packages (−18) for email → email-send: async-channel, async-imap, concurrent-queue, crossbeam-utils, equivalent, event-listener, event-listener-strategy, hashify, imap-proto, indexmap, mail-parser, minimal-lexical, parking, pin-project, pin-project-internal, pin-utils, self_cell, stop-token.

What is gated

Everything lives in src/providers/email_channel.rs and the two halves barely touch:

• Send (email-send, lettre) — create_smtp_transport, send_message, build_plain_message, build_message_with_attachment.
• Receive (email, async-imap + mail-parser) — connect_imap, fetch_unseen, wait_for_changes, listen_with_idle, run_idle_session, process_unseen, extract_sender, extract_text, ParsedEmail, IdleWaitResult, and the seen_messages dedupe field.

impl Channel for EmailChannel is gated on the full email feature. A send-only build cannot listen, so advertising the trait would promise a half-working channel. Nothing is lost: voice uses the inherent methods.

The module crate is unaffected — tinychannels-module already requests features = ["email", "lark", "whatsapp-web"] and still gets the complete provider.

A hole in the existing CI matrix, found while doing this

The --features email / --features lark lanes run at the workspace root, where tinychannels-module depends on this crate with features = ["email", "lark", "whatsapp-web"]. Cargo unifies those in, so those lanes have really been all-features builds and could never observe a gate being off. My first cargo check --features email-send "passed" for exactly that reason, and cargo tree still showed async-imap.

The three lanes added here use -p tinychannels --no-default-features, which is what makes them test the surface they name. I have deliberately not retrofitted the existing lanes in this PR — that is a separate change with its own risk of surfacing unrelated breakage, and worth doing on its own.

Tests

send_only_tests in email_channel.rs, gated #[cfg(all(test, feature = "email-send", not(feature = "email")))], pins the three items OpenHuman's voice path reaches for. This is the half a compile check cannot state on its own: the crate builds either way, so nothing would otherwise notice the send path growing a dependency on the receive half.

Verified locally, exit codes checked individually rather than chained:

fmt-check              PASS
clippy all-features    PASS
clippy email           PASS
clippy no-features     PASS
clippy email-send      PASS
test all-features      PASS
test email-send        PASS   (incl. both new tests, confirmed by name)
test email             PASS   (878 + 111 + 3)

The scoped clippy run caught four real items a plain cargo check did not — unused anyhow!, async_trait, Duration, and a seen_messages field never read in a send-only build. All are now gated.

Follow-up

Consumer side in OpenHuman: voice moves to tinychannels/email-send, and channels drops tinychannels/email once the providers run in the module.

Commits are the auto-commit hook's granular checkpoints, kept as-is by preference; the reasoning is here rather than squashed into one message.

Summary by CodeRabbit

• New Features

  • Added an email send-only configuration for SMTP delivery without requiring IMAP receiving capabilities.
  • The full email configuration continues to support both SMTP sending and IMAP receiving.
  • Email messages with plain text and attachments can now be built and formatted in send-only setups.

• Bug Fixes

  • Improved feature separation so send-only builds avoid unnecessary email-receiving components.
  • Added validation coverage for the new send-only email configuration.

[chatgpt-codex-connector]

Codex Review Summary

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Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-08-30T14:10:17.844237Z	e820aca	New commits

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[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro Plus

Run ID: 1eddd8ad-38b5-4a9e-b4d4-8ccdde311848

📥 Commits

Reviewing files that changed from the base of the PR and between 855e29c and fac79ca.

📒 Files selected for processing (4)

• .github/workflows/ci.yml
• Cargo.toml
• src/providers/email_channel.rs
• src/providers/mod.rs

Included review availability: Your plan provides up to 1 included review per hour; 0 remain after this review.

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Approval pending

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Use the checkbox below to review the latest commit. CodeRabbit will approve the changes if it finds no blocking issues.

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📝 Walkthrough

Walkthrough

The email feature is split into SMTP-only and full IMAP receive plus SMTP send modes. Receive code and state compile only with email. New tests and CI steps validate the SMTP-only mode.

Changes

Email feature split

Layer / File(s)	Summary
Feature contract and module exposure Cargo.toml, src/providers/mod.rs	email-send enables SMTP sending through lettre. email extends it with IMAP dependencies. The email module and EmailChannel re-export are available with email-send.
Conditional receive implementation src/providers/email_channel.rs	IMAP imports, state, helpers, IDLE processing, parsed email types, and receive test support require the email feature.
SMTP-only validation src/providers/email_channel.rs, .github/workflows/ci.yml	Send-only tests build plain and attachment messages. CI runs Clippy, build, and test commands with only email-send enabled.

Estimated code review effort: 3 (Moderate) | ~20 minutes

Merge Risk: ⚪ Minimal · up to fac79

The PR separates send-only email support from receive functionality and adds focused validation for the supported feature combinations; no actionable merge-blocking risk remains beyond normal checks and review.

Poem

A rabbit sends mail through a bright SMTP stream
IMAP stays tucked from the send-only dream
Plain notes and attachments hop into flight
CI checks each lane in the morning light
Two feature paths now compile just right

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 68.75% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 16 functions across 2 files. (2 skipped: …	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main change: splitting the full email provider from the SMTP-only email-send feature.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

Full details: Docstring Coverage

Explanation

Docstring coverage is 68.75% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 16 functions across 2 files. (2 skipped: 2 unsupported.)

✨ Finishing Touches 💡 2

📝 Generate docstrings 💡

• Create stacked PR
• Commit on current branch

🛠️ Fix failing CI checks 💡

• Create stacked PR
• Commit on current branch

---

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[tinysweeper]

Requesting changes: 1 lane(s) blocking, worst finding is high.

Fix or reply to the findings below and push. The next review clears this automatically once they are gone — you should not need to dismiss anything by hand.

             $0.0148 · 122,972 in / 4,352 out · 30,306 cached (25%) · openrouter/openai/text-embedding-3-small, deepseek/deepseek-v4-flash, z-ai/glm-5.2 · 342 embedded
critique:    $0.0066 · 53,691 in  / 2,917 out · 9,057 cached (17%)  · deepseek/deepseek-v4-flash, z-ai/glm-5.2
security:    $0.0055 · 46,505 in  / 857 out   · 15,331 cached (33%) · deepseek/deepseek-v4-flash, z-ai/glm-5.2
tests:       $0.0012 · 15,053 in  / 124 out   · 0 cached (0%)       · deepseek/deepseek-v4-flash
description: $0.0014 · 7,723 in   / 454 out   · 5,918 cached (77%)  · z-ai/glm-5.2

[tinysweeper]

How this change flows

2 changed behaviours across 8 relationships. 6 surrounding behaviours are shown (60 graph nodes walked). 40 further behaviours left out to keep the diagram readable.

flowchart LR
  n0["ParsedEmail<br/>changed<br/>1 finding"]:::blocking
  n1["yuanbao<br/>changed<br/>2 findings"]:::blocking
  n2["Result"]:::impacted
  n3["full_dm_dispatch"]:::impacted
  n4["new"]:::impacted
  n5["fetch_unseen"]:::impacted
  n6["process_unseen"]:::impacted
  n7["run_idle_session"]:::impacted
  n3 -->|uses| n1
  n5 -->|uses| n0
  n5 -->|uses| n2
  n5 -->|calls| n4
  n6 -->|uses| n2
  n6 -->|calls| n5
  n7 -->|uses| n2
  n7 -->|calls| n6
  classDef changed fill:#0d4429,stroke:#238636,color:#e6edf3
  classDef impacted fill:#161b22,stroke:#6e7681,color:#c9d1d9
  classDef flagged fill:#5a1e02,stroke:#d93f0b,color:#ffffff
  classDef blocking fill:#67060c,stroke:#f85149,color:#ffffff

Loading

_{Green: changed behaviour. Grey: surrounding behaviour. Arrows name the call, use, implementation, or test relationship. Orange: has findings. Red: has a finding that blocks the merge.}

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

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