Usage limits
Recipient counts, batch size, attachment size, and the daily sending quota.
| Limit | Value |
|---|---|
Recipients per email (to) | 1–50 |
cc / bcc recipients | up to 50 each |
Emails per batch (POST /emails/batch) | up to 100 |
| Attachment size (per file) | up to 25 MB |
| Total attachments per email | up to 40 MB |
| Daily sending quota | per-account (defaults to a conservative cap; raised on request) |
When a send would exceed your daily quota, the request is rejected with a 429 daily_quota_exceeded — it is not queued or partially sent. Wait until the quota resets (or request a higher cap) and retry.
Rate limits
The send endpoints (POST /emails and the transactional send API) are rate-limited per client IP to absorb bursts — on the order of a few dozen send requests per minute. Exceeding that rate returns a 429 rate_limit_exceeded response. Other endpoints are not subject to this per-request cap; your sustained sending volume is governed by your account quotas (below) rather than a strict per-second limit.
To stay within the limit, reduce the rate at which you call the send endpoints — for example by introducing a queue or lowering the number of concurrent requests — and honor the retry-after header when you receive a 429.
Rate-limit response headers
Responses from the rate-limited send endpoints include headers describing your current rate-limit window (the draft-6 lowercase form of the IETF RateLimit Headers draft):
| Header | Description |
|---|---|
ratelimit-limit | Maximum number of requests allowed within a window. |
ratelimit-remaining | How many requests you have left within the current window. |
ratelimit-reset | How many seconds until the limits are reset. |
retry-after | How many seconds you should wait before making a follow-up request. |
Email quota headers
In addition to rate limits, responses return headers that track your email sending quotas:
| Header | Description |
|---|---|
x-mailblastr-daily-quota | Your used daily email sending quota. Only sent to accounts subject to a daily cap. |
x-mailblastr-monthly-quota | Your used monthly email sending quota. |
When you exceed a quota you receive a 429 with either daily_quota_exceeded (reached your daily email quota — wait until 24 hours have passed or upgrade) or monthly_quota_exceeded (reached your monthly email quota — upgrade to increase it). Both sent and received emails count towards these quotas.