Limits
Technical limits that apply on every plan. For monthly send volumes by plan, see Pricing.
Send quotas
Monthly send volumes depend on your plan — see Pricing. Preview accounts have additional per-minute and daily caps:
| Account type | Per-minute | Daily | Monthly |
|---|---|---|---|
Preview account (bx_test_* key) | 10 requests | 50 emails | 1,500 emails |
| Full account (free or paid) | — | — | See Pricing |
When a quota is reached the API returns 429. See Rate limits for error shapes and retry guidance.
Per-send limits
| Limit | Value |
|---|---|
Recipients per message (to + cc + bcc) | 50 |
| Attachments per message | 10 |
| Single attachment size | 5 MB |
| Total attachments per message | 7 MB |
Exceeding these returns a 400 error. See Error codes for the full list (recipient_limit_exceeded, attachment_too_large, too_many_attachments).
Batch limits
| Limit | Value |
|---|---|
| Messages per batch request | 100 |
| Total recipients across a batch | 1,000 |
Idempotency
| Limit | Value |
|---|---|
Idempotency-Key max length | 256 bytes |
| Idempotency window | 24 hours |
Attachment types
Only the following MIME types are accepted:
| Category | Formats |
|---|---|
| Documents | PDF, Word (.doc, .docx), Excel (.xls, .xlsx), PowerPoint (.ppt, .pptx), CSV, plain text |
| Images | PNG, JPEG, GIF, WebP |
| Calendar | ICS |
Any other type returns 400 attachment_type_not_allowed.
What’s next
| You want to… | Go to |
|---|---|
| See monthly volumes and plan prices | Pricing |
| Understand 429 errors and retry behaviour | Rate limits |
| See all API error codes | Error codes |
| Make your first send | Quickstart |
Start sending
9,000 transactional emails a month, free.
No credit card. No commitment. Most integrations never come close to these limits.