Mailpro is built to scale — from your first 100 contacts to a million-strong list with daily transactional volume. The platform has practical limits that protect deliverability, performance and your sender reputation. This page lists the limits that matter day-to-day, why each one exists, and where to upgrade when you outgrow your current plan.
Account-wide structural limits
| Resource | Limit | Why |
|---|---|---|
| Custom contact fields | 30 per account | Performance + clean schema |
| Lists per account | Unlimited | No technical reason to cap |
| Tags per contact | Unlimited | Tags are cheap |
| Segments per list | 100 | Beyond this, simplify your taxonomy |
| Webhooks per account | 10 | Each pings your endpoint per event |
Per-message limits
| Limit | Value | Notes |
|---|---|---|
| SMTP message size | 20 MB | Includes Base64-encoded attachments |
| Recipients per SMTP message | 50 | To+CC+BCC combined |
| HTML body weight (recommended) | < 100 KB | To avoid Gmail message clipping |
| Image / file upload | 5 MB / 10 MB | Per file type |
Daily sending limits
Daily limits depend on your plan and account warm-up history, not a fixed number. New accounts ramp up over the first weeks; established accounts can send millions per day. The platform throttles automatically to protect your IP reputation if engagement drops or complaints spike.
API and rate limits
| Endpoint | Rate limit | Burst |
|---|---|---|
| SMTP | Per plan, throttled | Auto-managed |
| REST API | 60 req/min default | Higher on enterprise plans |
| Webhook delivery | Best effort, retried 5x | Backoff: 1m, 5m, 15m, 1h, 6h |
What happens when you hit a limit
For most limits, Mailpro shows a clear error in the UI or returns a structured API response (e.g. HTTP 429 with retry-after header). For deliverability-sensitive limits (sending too fast, too many recipients), the platform queues and paces automatically rather than failing. You always know what happened — nothing fails silently.
Limits keep things fast and deliverable
Most users never notice the limits. If you do, your business has scaled enough to consider an enterprise plan. See pricing, the limits FAQ and SMTP message limits.