Send emails with Go

package main

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

const flaresendURL = "https://mailer.example.com"

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

type SendResult struct {
	ID         string `json:"id"`
	Status     string `json:"status"`
	Idempotent bool   `json:"idempotent,omitempty"`
}

type APIError struct {
	Type    string `json:"type"`
	Code    string `json:"code"`
	Message string `json:"message"`
	Param   string `json:"param,omitempty"`
	Status  int    `json:"-"`
}

func (e *APIError) Error() string { return fmt.Sprintf("%d %s: %s", e.Status, e.Code, e.Message) }

var client = &http.Client{Timeout: 30 * time.Second}

func send(email SendEmail, idempotencyKey string) (*SendResult, error) {
	body, err := json.Marshal(email)
	if err != nil {
		return nil, err
	}
	req, err := http.NewRequest("POST", flaresendURL+"/v1/emails", bytes.NewReader(body))
	if err != nil {
		return nil, err
	}
	req.Header.Set("Authorization", "Bearer "+os.Getenv("FLARESEND_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 nil, err
	}
	defer res.Body.Close()

	if res.StatusCode >= 300 {
		var wrapper struct {
			Error APIError `json:"error"`
		}
		if err := json.NewDecoder(res.Body).Decode(&wrapper); err != nil {
			return nil, fmt.Errorf("flaresend: HTTP %d", res.StatusCode)
		}
		wrapper.Error.Status = res.StatusCode
		return nil, &wrapper.Error
	}

	var result SendResult
	if err := json.NewDecoder(res.Body).Decode(&result); err != nil {
		return nil, err
	}
	return &result, nil
}

func main() {
	result, err := send(SendEmail{
		From:    "Acme <hello@acme.com>",
		To:      []string{"ada@example.com"},
		Subject: "Hello from Go",
		HTML:    "<p>It works.</p>",
		Text:    "It works.",
	}, "hello-go-1")
	if err != nil {
		fmt.Println("send failed:", err)
		os.Exit(1)
	}
	fmt.Println(result.ID, result.Status) // email_01K6B2Y4ZP9R3M7T8V5N2QXW4C queued
}

{
  "error": {
    "type": "unprocessable",
    "code": "recipient_suppressed",
    "message": "ada@example.com is on the suppression list (hard_bounce)",
    "param": "ada@example.com"
  }
}

var apiErr *APIError
if errors.As(err, &apiErr) && apiErr.Code == "recipient_suppressed" {
	// don't retry; the address bounced or complained before
}