A transactional email API lets your application send individual, triggered emails such as password resets, receipts, order confirmations and verification codes, programmatically instead of by hand. You can send transactional email two ways: over an SMTP relay (works in any language over port 587 or 465) or over an HTTP API. Both are "an API" in the sense that matters: your code sends the mail. Bulko sends transactional email over its SMTP relay from $5 for 5,000 emails with no subscription, and offers an HTTP API for campaign automation. Deliverability for transactional mail comes from authentication (SPF, DKIM, DMARC) and a clean sending domain, not from the transport you choose.
This guide explains what a transactional email API is, the difference between SMTP and HTTP sending, and how to send transactional email with Bulko.
What is a transactional email API?
Transactional email is mail triggered by a user action and sent to one person: a signup confirmation, a password reset, a payment receipt, a shipping update. It is different from bulk or marketing email, which sends one message to a list. A transactional email API is simply the interface your application uses to send those messages automatically. That interface can be SMTP (a mail protocol every language supports) or an HTTP/REST API (a web request), and dedicated providers usually offer both.
SMTP vs HTTP for transactional email
SMTP relay. Your app connects to a mail server over port 587 (TLS / STARTTLS) or 465 (SSL) with a username and password and hands off the message. Every language and framework already speaks SMTP, so there is nothing new to learn and no vendor SDK to adopt. This is the simplest path and the most portable.
HTTP / REST API. Your app sends a JSON web request with a Bearer token. This adds provider-specific features such as templating, per-message webhooks and detailed event tracking, at the cost of tying your code to that vendor's API shape.
For most transactional needs, SMTP is enough: it is reliable, standard, and switching providers means changing only host and credentials.
Transactional email providers compared
| Provider | Best for | Send methods | Entry |
|---|---|---|---|
| Bulko | Cheap pay-as-you-go transactional over SMTP | SMTP (587/465), HTTP campaign API | $5 (5,000 emails, one-time) |
| Postmark | HTTP-first transactional, templates, fast delivery | SMTP + REST API | from ~$15/mo |
| SendGrid | Full platform, marketing + transactional | SMTP + REST API | from ~$19.95/mo |
| Mailgun | Developer HTTP API, routing, logs | SMTP + REST API | from ~$15/mo |
| Amazon SES | Lowest per-email price at scale | SMTP + AWS SDK API | ~$0.10 / 1,000 |
Details were checked against public plans in 2026. Providers change plans often, so confirm current pricing and limits before committing.
Sending transactional email with Bulko
Bulko sends transactional email over its SMTP relay, which works from any language or framework without an SDK:
- SMTP: ports 587 (TLS / STARTTLS) and 465 (SSL), LOGIN / PLAIN over TLS. Drop the host, port and credentials into your existing mailer.
- HTTP API: an authenticated API (Bearer token issued in the dashboard) at
/api/v1handles campaign automation, subscriber import, replies and account balance. - Pricing: from $5 for 5,000 emails; $15 / 20,000; $30 / 50,000; $60 / 150,000. Pay-as-you-go, no subscription, credits do not expire.
- Safety: every message is scanned by an AI content filter; blocked messages return an SMTP error and do not consume credit.
- Payment: card and crypto.
Bulko is the strongest fit when you want cheap, no-commitment transactional sending over standard SMTP. If you need HTTP-first features such as built-in templating engines and rich per-message webhooks, a dedicated transactional API like Postmark is purpose-built for that.
How to keep transactional email out of spam
- Authenticate the sending domain with SPF, DKIM and DMARC. Gmail and Yahoo rate-limit or reject unaligned mail in 2026.
- Send from a domain you control, not a free mailbox address.
- Use a real reply-to and a clear from name so receipts and resets look legitimate.
- Keep transactional and marketing streams separate so a marketing complaint does not sink your password resets.
Frequently asked questions
What is a transactional email API?
It is the interface an application uses to send triggered, one-to-one emails (password resets, receipts, confirmations) programmatically. It can be an SMTP relay or an HTTP/REST API; both let your code send the mail automatically.
Is SMTP a transactional email API?
Yes, in practice. SMTP is the standard mail protocol every language supports, and sending transactional email over an SMTP relay is a fully valid, portable API. Many providers, including Bulko, send transactional mail this way.
What is the cheapest transactional email API?
For low or irregular volume, a pay-as-you-go relay with no monthly minimum such as Bulko ($5 for 5,000 emails) is usually cheapest. For very high steady volume, Amazon SES has the lowest per-email price in exchange for more setup.
Can I send transactional email over SMTP with Bulko?
Yes. Bulko SMTP Access works over port 587 (TLS / STARTTLS) or 465 (SSL) with a username and password, in any SMTP-capable language or framework, from $5 for 5,000 emails.
What is the difference between transactional and bulk email?
Transactional email is triggered by a user action and sent to one recipient (a receipt, a reset). Bulk email sends one message to a whole list (a newsletter, a campaign). They should use separate sending streams to protect deliverability.
See Bulko SMTP Access, or read our guides on the best SMTP services and the cheapest SMTP relays.
