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Railmail limits requests to 60 per minute per API key. The limit is applied per key, so separate keys have separate budgets.

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Rate limit headers

Every response includes headers describing your current budget:
HeaderDescription
X-RateLimit-LimitThe maximum number of requests allowed per window (60).
X-RateLimit-RemainingThe number of requests remaining in the current window.
X-RateLimit-ResetWhen the current window resets, so you know when Remaining refills.
Read these headers to pace your requests before you hit the limit.

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When you exceed the limit

Once you exceed the limit, the API returns 429 Too Many Requests with a Retry-After header telling you how many seconds to wait before retrying: 
HTTP/1.1 429 Too Many Requests
Retry-After: 42
Content-Type: application/problem+json

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Handling limits gracefully

1

Watch X-RateLimit-Remaining

Slow down as Remaining approaches zero rather than sending requests until you’re rejected.
2

Respect Retry-After on 429

When you get a 429, wait the number of seconds in Retry-After before retrying — don’t retry immediately.
3

Back off on repeated 429s

If you keep hitting the limit, add exponential backoff and consider batching work or spreading it over time.
For bulk work like importing many subscribers, add a small delay between requests to stay comfortably under 60/minute instead of bursting and getting throttled.