EffiLink API Rate Limits and SMTP Throttling

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SMTP rate limits

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Web API rate limits

• A per-hour limit caps the total number of requests you can make to that endpoint within any rolling 60-minute window.
• A per-second limit caps the instantaneous burst rate to prevent traffic spikes.
• Where both apply, both limits must be satisfied simultaneously.

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Handling HTTP 429 Too Many Requests

HTTP/1.1 429 Too Many Requests

1. Detect the 429 — Check the HTTP status code on every response.
2. Back off — Wait before retrying. Start with a short delay (e.g. 1 second) and increase it exponentially on repeated 429 responses (e.g. 2 s, 4 s, 8 s).
3. Respect the reset window — If the response includes a Retry-After header, wait at least that many seconds before your next attempt.
4. Resume — Once the rate window resets, resume normal request flow.

// Example: simple exponential back-off
async function sendWithRetry(payload, maxRetries = 5) {
  let delay = 1000; // start at 1 second
  for (let attempt = 0; attempt < maxRetries; attempt++) {
    const res = await fetch('https://api.effilink.co/v5/sms/sends', {
      method: 'POST',
      headers: { 'ApiKey': 'YOUR_API_KEY', 'Content-Type': 'application/json' },
      body: JSON.stringify(payload),
    });
    if (res.status !== 429) return res;
    await new Promise(r => setTimeout(r, delay));
    delay *= 2; // double the wait time on each retry
  }
  throw new Error('Max retries reached');
}

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Tips for staying within limits

• Batch your sends. The /v5/sms/sends endpoint accepts up to 100 recipients per request. Grouping recipients reduces the total number of API calls required. See Batch Personalized SMS for details.
• Schedule bulk operations off-peak. If you need to make a high volume of calls (e.g. syncing contacts or triggering campaigns), spread them over time rather than issuing them all at once.
• Cache read responses. For endpoints like /v5/verified_senders or /v5/contacts, cache the results locally and refresh only when your data changes rather than polling on a tight loop.
• Monitor your usage. Keep track of your request rate in your application layer so you can proactively throttle before hitting platform limits rather than reacting to 429 errors.
• Use the transactional endpoint judiciously. At 10 requests/second, /v5/transactional has a strict burst cap. Queue outgoing transactional messages and dispatch them at a controlled rate to avoid unexpected rejections.