Litmus

Litmus is the long-standing standard for previewing how a marketing or transactional email actually renders across Gmail, Outlook, Apple Mail, mobile clients, and dark-mode variants. Pre-send checklist catches broken links, missing alt text, spam triggers, image weight, and load time. Build, Proof, and Analytics modules cover authoring, review, and post-send heatmaps.

An agent stack treats email as a two-way primitive. Inbound has to arrive as structured JSON a tool call can consume, sends have to be safe to retry when a model loop repeats itself, and every action needs an audit trail. A provider without inbound parsing only solves half the problem.

Against inbound parsing into tool calls and model-generated sends that must not duplicate, Litmus covers webhook coverage and multi-tenant support, and does not cover inbound parsing, idempotency keys, event stream, batch sending, and SDK breadth. Whether that gap matters depends on how central those are to the build.

• Inbound parsing: No
• Idempotency keys: No
• Webhooks: Yes
• Operating since: Not published
• Event stream: No
• Best published rate per 1,000: Tiers not published in comparable form
• Batch sending: No
• SDKs: None
• Multi-tenant: Yes

Not applicable; preview and QA, not a sender.

Marketing and lifecycle teams that need pixel-accurate previews across every major client before sending.

Solo developers sending only plain-text or simple transactional email.