Both SMTP and a transactional Email API send messages from your application to your customers, but they take different paths and offer different trade-offs. Choosing the right one depends on what you already have in place, the volume you send and how deeply you need to integrate with the email lifecycle (opens, clicks, bounces, webhooks). Mailpro offers both — this guide helps you pick.
How each works
SMTP relies on a 40-year-old protocol: your app opens a TCP connection, authenticates, hands the message over, the server queues it. Email APIs are modern HTTPS endpoints: your app does an HTTP POST with a JSON payload describing recipient, subject, body and metadata, and gets a tracking ID back instantly.
Side-by-side comparison
| Aspect | SMTP relay | Email API |
|---|---|---|
| Setup | Username + password in existing email settings | HTTPS endpoint + API token + small SDK or curl |
| Latency | 100–500 ms per message | 30–100 ms per message |
| Throughput | Limited by SMTP connection pool | Massively parallel HTTP |
| Templating | Inside your message HTML | Server-side templates with variables |
| Real-time events | Polling or none | Webhooks for delivered, opens, clicks, bounces |
| Bounce handling | Async via SMTP codes + DSNs | JSON event in webhook |
| Retry / queueing | Built into SMTP | You must build it in your client |
| Best for | Drop-in replacement for an existing mailer | New apps that want full visibility |
When to choose SMTP
Existing systems (Magento, WooCommerce, WordPress, Microsoft Exchange, Salesforce, Odoo) all support SMTP out of the box. Switching to Mailpro’s SMTP relay is usually a five-minute config change — no code, no deploys.
When to choose the API
If you are building a SaaS, a custom mobile app or a notification service, the API gives you everything in one HTTP call: send, attach, schedule, get the message ID, subscribe to webhooks. Watch the how to use the API tutorial.
Cost
Mailpro charges per email regardless of the channel. The same plan covers SMTP and API, so you can mix freely without worrying about double billing. Read the full breakdown in our SMTP relay vs Email API blog post.
Weighing an email API like Amazon SES against a managed platform? See our comparison of the best Amazon SES alternatives.
Both protocols, one platform
Mailpro’s Email API and SMTP relay share the same authenticated IPs, deliverability stats and account credits. Read the SMTP vs API FAQ.