Go

SpacePost is a plain JSON API, so the standard library is enough.

main.go

package main

import (
  "bytes"
  "encoding/json"
  "fmt"
  "net/http"
  "os"
  "time"
)

type Email struct {
  From    string   `json:"from"`
  To      []string `json:"to"`
  Subject string   `json:"subject"`
  HTML    string   `json:"html,omitempty"`
  Text    string   `json:"text,omitempty"`
}

type apiError struct {
  StatusCode int    `json:"statusCode"`
  Name       string `json:"name"`
  Message    string `json:"message"`
}

func (e apiError) Error() string { return fmt.Sprintf("%d %s: %s", e.StatusCode, e.Name, e.Message) }

var client = &http.Client{Timeout: 15 * time.Second}

func Send(e Email, idempotencyKey string) (string, error) {
  payload, _ := json.Marshal(e)
  req, err := http.NewRequest("POST", "https://api.spacepost.dev/emails", bytes.NewReader(payload))
  if err != nil {
    return "", err
  }
  req.Header.Set("Authorization", "Bearer "+os.Getenv("SPACEPOST_API_KEY"))
  req.Header.Set("Content-Type", "application/json")
  if idempotencyKey != "" {
    req.Header.Set("Idempotency-Key", idempotencyKey)
  }
  res, err := client.Do(req)
  if err != nil {
    return "", err
  }
  defer res.Body.Close()

  if res.StatusCode >= 300 {
    var ae apiError
    if err := json.NewDecoder(res.Body).Decode(&ae); err != nil {
      return "", fmt.Errorf("unexpected status %d", res.StatusCode)
    }
    return "", ae
  }
  var ok struct {
    ID string `json:"id"`
  }
  if err := json.NewDecoder(res.Body).Decode(&ok); err != nil {
    return "", err
  }
  return ok.ID, nil
}

func main() {
  id, err := Send(Email{
    From:    "Acme <hello@acme.com>",
    To:      []string{"delivered@test.spacepost.dev"},
    Subject: "Hello from Go",
    HTML:    "<p>It works.</p>",
  }, "")
  if err != nil {
    panic(err)
  }
  fmt.Println("sent", id)
}

SPACEPOST_API_KEY=sp_live_YOUR_API_KEY go run .