Email verification API in Go

// Package simpleverifier checks email addresses with the SimpleVerifier API.
package simpleverifier

import (
	"context"
	"encoding/json"
	"fmt"
	"net/http"
	"net/mail"
	"net/url"
	"os"
	"time"
)

const apiURL = "https://simpleverifier.com/api/v1/verify"

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

// Result is the JSON body the verify endpoint returns.
type Result struct {
	Email          string `json:"email"`
	Status         string `json:"status"` // "valid", "risky" or "invalid"
	Reason         string `json:"reason"`
	MailboxChecked bool   `json:"mailbox_checked"`
}

// EmailStatus returns "valid", "risky" or "invalid". When the API could not
// answer it returns "" and an error; callers should let the address through.
func EmailStatus(ctx context.Context, email string) (string, error) {
	addr, err := mail.ParseAddress(email)
	if err != nil || addr.Address != email {
		return "invalid", nil // fails the local syntax check, no API call made
	}

	req, err := http.NewRequestWithContext(ctx, http.MethodGet, apiURL+"?email="+url.QueryEscape(email), nil)
	if err != nil {
		return "", err
	}
	req.Header.Set("Authorization", "Bearer "+os.Getenv("SIMPLEVERIFIER_KEY"))

	res, err := client.Do(req)
	if err != nil {
		return "", err // timeout or network error
	}
	defer res.Body.Close()

	if res.StatusCode != http.StatusOK {
		return "", fmt.Errorf("simpleverifier: HTTP %d", res.StatusCode) // 401, 402, 429, 503
	}

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

package main

import (
	"log"
	"net/http"
	"strings"

	"example.com/app/simpleverifier"
)

func signupHandler(w http.ResponseWriter, r *http.Request) {
	email := strings.TrimSpace(r.FormValue("email"))

	status, err := simpleverifier.EmailStatus(r.Context(), email)
	if err != nil {
		log.Printf("simpleverifier unavailable, accepting without a mailbox check: %v", err)
	}
	if status == "invalid" {
		http.Error(w, "This email address cannot receive mail. Please check it.", http.StatusUnprocessableEntity)
		return
	}

	// Save the signup here. status is "valid", "risky" or "" (API unavailable).
	w.WriteHeader(http.StatusCreated)
}

func main() {
	http.HandleFunc("POST /signup", signupHandler)
	log.Fatal(http.ListenAndServe(":8080", nil))
}

{
  "email": "jane@company.com",
  "status": "valid",
  "reason": "Mailbox exists",
  "mailbox_checked": true
}