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.

Judged purely as an API. What the request contract guarantees on retry, how complete the webhook event coverage is, whether suppressions are readable and writable programmatically, and how much the message log retains when something needs debugging in production.

Against high-frequency programmatic sends where a missed delivery is a product failure, Litmus covers webhook coverage, and does not cover idempotency keys, event stream, the suppressions API, batch sending, and SDK breadth. Whether that gap matters depends on how central those are to the build.

• Idempotency keys: No
• Webhooks: Yes
• Event stream: No
• Operating since: Not published
• Suppressions API: No
• Best published rate per 1,000: Tiers not published in comparable form
• Batch sending: No
• SDKs: None
• Deliverability: Not applicable; preview and QA, not a sender.

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.