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The Apollo Signal Knowledge Base is a standalone support library for people who operate email in production. It explains observable product behavior, exact API boundaries, and diagnostic steps you can verify.
Every product fact in this library comes from the current Signal customer dashboard, public API contract, or active service behavior. When a value depends on your plan or deployment, the article points you to the live dashboard or endpoint instead of publishing a guessed number.

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Start with the symptom

A send returned 201

Understand what Signal accepted and which delivery evidence still has to arrive.

A message has an unfamiliar status

Read every public email state, including sending and indeterminate.

A domain will not verify

Work through authoritative DNS, host formatting, values, and proxying.

A recipient was blocked

Identify manual, bounce, complaint, and unsubscribe suppression state.

A webhook failed

Inspect attempts, automatic retries, and controlled replay behavior.

An API call failed

Separate validation, authentication, permission, conflict, and rate-limit failures.

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Browse by system boundary

  • Sending and delivery covers request admission, recipient expansion, scheduling, idempotency, test mode, attachments, statuses, and event streams.
  • Domains and DNS covers records, verification, automatic setup, Cloudflare, regions, tracking, DMARC, BIMI, and deliverability evidence.
  • Contacts and consent covers contacts, custom properties, segments, topics, global unsubscribe state, and suppressions.
  • Webhooks and observability covers event types, signatures, retries, replays, metrics, logs, and API-key usage.
  • Security and integrations covers API keys, SMTP, and provider migration.

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Facts, diagnostics, and decisions

A fact describes a current Signal contract, such as the 50-recipient audience ceiling or the name of a signature header. A diagnostic step tells you how to observe state, such as checking the message timeline or public DNS. A decision remains yours, such as whether to use a root domain or a subdomain. Decision articles identify the product consequences without presenting a preference as a platform guarantee.

Read the API reference

Use the API Reference for exact request fields, response schemas, permissions, and status codes.