Send Email (SMTP and Resend)

Category: Backend & APIs

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One pack, two ways to send transactional email from an Air Pipe route — pick whichever fits your stack:

SMTP (smtp.yml) — Air Pipe's native email: action speaks SMTP directly (via STARTTLS). Works with any mail server: Gmail, Fastmail, Postmark, Mailgun, Amazon SES SMTP, or your own relay. No SDK, self-hosted-friendly.
Resend (resend.yml) — Air Pipe calls the Resend HTTP API. No SMTP server to run; just an API key and a verified domain.

Both routes take the same request shape — { to, subject, html } — so you can switch providers without changing your callers.

Which one should I use?

	SMTP (`smtp.yml`)	Resend (`resend.yml`)
How	Native `email:` action opens an SMTP connection	HTTP POST to `api.resend.com`
Best for	Existing mail infra, self-hosted, any provider	Fastest setup, no server to manage
You need	SMTP host + user + pass	A Resend API key + verified domain
Leaves your infra	Only the SMTP connection you configure	HTTPS call to Resend

Deploy one file for real use — they're independent. Both ship here (distinct /email/smtp and /email/resend routes) so you can compare.

Endpoints

Method	Route	Sends via	Body
`POST`	`/email/smtp`	Native SMTP	`{ to, subject, html }`
`POST`	`/email/resend`	Resend API	`{ to, subject, html }`

Both validate the body (all three fields required → 400), send, and return { status: "sent", provider, … }. A delivery failure returns 502.

Setup

Add the managed variables for the option you're using (Air Pipe dashboard, or ap_vars self-hosted):

SMTP (smtp.yml):

Name	Value
`SMTP_HOST`	SMTP server hostname (e.g. `smtp.postmarkapp.com`)
`SMTP_USER`	SMTP username
`SMTP_PASS`	SMTP password / app password / API-key-as-password
`SMTP_FROM`	Verified sender — plain or named (`Acme <[email protected]>`)

The smtp: block defaults to port 587 + STARTTLS. For implicit TLS use port: 465; for an internal unencrypted relay use port: 25 + tls: false. port and tls are literal config values (not ap_vars), so edit them in smtp.yml directly.

Resend (resend.yml):

Name	Value
`RESEND_API_KEY`	Your Resend API key (starts with `re_`)
`RESEND_FROM`	Verified sender (`Acme <[email protected]>`)

Quick start

BASE=https://your-airpipe-host

# Send via SMTP
curl -sX POST $BASE/email/smtp -H 'content-type: application/json' -d '{
  "to": "[email protected]",
  "subject": "Welcome to Acme",
  "html": "<h1>Welcome!</h1><p>Thanks for signing up.</p>"
}'
# → { "to": "...", "subject": "...", "status": "sent", "provider": "smtp" }

# Send via Resend — identical body
curl -sX POST $BASE/email/resend -H 'content-type: application/json' -d '{
  "to": "[email protected]",
  "subject": "Welcome to Acme",
  "html": "<h1>Welcome!</h1><p>Thanks for signing up.</p>"
}'
# → { "id": "…", "status": "sent", "provider": "resend" }

Notes

Same body, two providers — swap smtp ↔ resend in the path; callers don't change.
Quotes & newlines are safe — the Resend route runs every user value through ->json_escape, so a " or newline in the subject/body can't break the JSON payload.
HTML email — both routes take an html body. (The native SMTP action also supports a text: field for a plaintext part; add it to smtp.yml if you want multipart.)
AWS SES — the native action reserves an aws_ses option, but SMTP and Resend are the two supported paths today; for SES, use its SMTP interface with smtp.yml.

Configuration

resend.yml

name: SendEmailResend
description: Send transactional email via the Resend HTTP API — the managed, no-SMTP-server alternative. Air Pipe calls Resend's REST endpoint directly with your API key.

docs: true

# Resend (https://resend.com) is a developer-friendly email API: no SMTP server
# to run, just an API key and a verified sending domain. This is the managed
# option — Air Pipe POSTs to Resend over HTTPS instead of opening an SMTP socket.
#
# Free tier: 100 emails/day (3,000/month). Verify your domain in the Resend
# dashboard before sending from it.
#
# Required managed variables:
#   RESEND_API_KEY — your Resend API key (starts with "re_")
#   RESEND_FROM    — verified sender, plain or named ("Acme <[email protected]>")

interfaces:

  # POST /email/resend
  # Body: { "to": "...", "subject": "...", "html": "<...>" }
  email/resend:
    output: http
    method: POST
    summary: Send email via Resend
    description: Send an HTML email through the Resend API. Same request shape as the SMTP route.
    tags: [email, resend]
    request_example:
      to: [email protected]
      subject: Welcome to Acme
      html: "<h1>Welcome!</h1><p>Thanks for signing up.</p>"
    response_example:
      id: 4ef9a417-02e9-4d39-ad75-9611e0fcc33c
      status: sent
      provider: resend

    actions:
      - name: Validate
        input: a|body|
        hide_data_on_success: true
        assert:
          http_code_on_error: 400
          error_message: "to, subject, and html are required"
          tests:
            - value: to
              is_not_null: true
              is_not_empty: true
              description: Recipient email address.
            - value: subject
              is_not_null: true
              is_not_empty: true
              description: Email subject line.
            - value: html
              is_not_null: true
              is_not_empty: true
              description: HTML body of the email.

      # Every user-supplied value goes through ->json_escape so a quote or newline
      # in the subject/body can't break the JSON payload sent to Resend.
      - name: SendResend
        run_when_succeeded:
          actions: [Validate]
          http_code_on_error: 400
        http:
          url: https://api.resend.com/emails
          method: POST
          headers:
            content-type: application/json
            authorization: "Bearer a|ap_var::RESEND_API_KEY|"
          body: |
            {
              "from": "a|ap_var::RESEND_FROM|",
              "to": ["a|Validate::to->json_escape|"],
              "subject": "a|Validate::subject->json_escape|",
              "html": "a|Validate::html->json_escape|"
            }
        assert:
          http_code_on_error: 502
          error_message: "Resend rejected the email"
          tests:
            - value: status
              is_equal_to: 200

      - name: Result
        run_when_succeeded: [SendResend]
        input: a|SendResend|
        post_transforms:
          - extract_value: body
          - add_attribute:
              status: sent
              provider: resend

smtp.yml

name: SendEmailSmtp
description: Send transactional email through any SMTP server (Gmail, Fastmail, Postmark, Mailgun, Amazon SES SMTP, or your own relay) using Air Pipe's native email action — no provider SDK, no HTTP client to wire up.

docs: true

# Air Pipe's `email:` action speaks SMTP directly, so you only need your
# provider's host + credentials — no SDK. This is the self-hosted-friendly
# option: any mail server works, and nothing leaves your infrastructure except
# the SMTP connection you configure.
#
# TLS / PORTS (set in the `smtp:` block below):
#   587 + tls: true   → STARTTLS  (recommended default — used here)
#   465               → implicit TLS / SMTPS
#   25  + tls: false  → unencrypted (internal relays / testing only)
#
# Required managed variables:
#   SMTP_HOST — SMTP server hostname   (e.g. smtp.gmail.com, smtp.postmarkapp.com)
#   SMTP_USER — SMTP username
#   SMTP_PASS — SMTP password / app password / API-key-as-password
#   SMTP_FROM — verified sender, plain or named ("Acme <[email protected]>")

interfaces:

  # POST /email/smtp
  # Body: { "to": "...", "subject": "...", "html": "<...>" }
  email/smtp:
    output: http
    method: POST
    summary: Send email via SMTP
    description: Send an HTML email through your SMTP server using the native email action.
    tags: [email, smtp]
    request_example:
      to: [email protected]
      subject: Welcome to Acme
      html: "<h1>Welcome!</h1><p>Thanks for signing up.</p>"
    response_example:
      status: sent
      provider: smtp
      to: [email protected]

    actions:
      - name: Validate
        input: a|body|
        hide_data_on_success: true
        assert:
          http_code_on_error: 400
          error_message: "to, subject, and html are required"
          tests:
            - value: to
              is_not_null: true
              is_not_empty: true
              description: Recipient email address.
            - value: subject
              is_not_null: true
              is_not_empty: true
              description: Email subject line.
            - value: html
              is_not_null: true
              is_not_empty: true
              description: HTML body of the email.

      # The native email action opens an SMTP connection and delivers the message.
      # a|...| directives interpolate into the string fields (not into `port`/`tls`,
      # which must stay literal because the config is typed).
      - name: SendSmtp
        run_when_succeeded:
          actions: [Validate]
          http_code_on_error: 400
        email:
          smtp:
            server: a|ap_var::SMTP_HOST|
            user: a|ap_var::SMTP_USER|
            pass: a|ap_var::SMTP_PASS|
            port: 587
            tls: true
          from: a|ap_var::SMTP_FROM|
          to: a|Validate::to|
          subject: a|Validate::subject|
          html: a|Validate::html|

      - name: Result
        run_when_succeeded:
          actions: [SendSmtp]
          http_code_on_error: 502
        input: a|Validate|
        post_transforms:
          - remove_keys: [html]
          - add_attribute:
              status: sent
              provider: smtp

Send Email (SMTP and Resend)

Which one should I use?​

Endpoints​

Setup​

Quick start​

Notes​

Configuration​

resend.yml​

smtp.yml​