Go net/http

1 Keep the key on the server

MAILVET_KEY=mv_live_your_key

Set it wherever your server reads its environment. Never put it in client code.

2 The whole server

main.go

package main

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

type check struct {
	Verdict    string `json:"verdict"`
	Disposable bool   `json:"disposable"`
	DidYouMean string `json:"did_you_mean"`
}

// MailVet gives DNS up to 4 seconds, so 5 covers a slow domain.
var client = &http.Client{Timeout: 5 * time.Second}

// lookup returns nil when MailVet gives no answer.
func lookup(ctx context.Context, email string) *check {
	q := url.Values{"email": {email}}
	req, err := http.NewRequestWithContext(ctx, http.MethodGet,
		"https://mailvet.dev/v1/check?"+q.Encode(), nil)
	if err != nil {
		log.Printf("mailvet: %v", err)
		return nil
	}
	req.Header.Set("Authorization", "Bearer "+os.Getenv("MAILVET_KEY"))

	res, err := client.Do(req)
	if err != nil {
		log.Printf("mailvet: no answer: %v", err)
		return nil
	}
	defer res.Body.Close()
	if res.StatusCode != http.StatusOK {
		log.Printf("mailvet: HTTP %d", res.StatusCode)
		return nil
	}
	var c check
	if err := json.NewDecoder(res.Body).Decode(&c); err != nil {
		log.Printf("mailvet: bad answer: %v", err)
		return nil
	}
	return &c
}

// problem says what to tell the user, or "" if the address can pass.
func problem(email string, c *check) string {
	switch {
	case c == nil:
		return "" // a signup shouldn't die because of us
	case c.DidYouMean != "":
		local := email[:strings.LastIndex(email, "@")]
		return fmt.Sprintf("Did you mean %s@%s?", local, c.DidYouMean)
	case c.Verdict == "invalid":
		return "That address can't receive mail."
	case c.Verdict == "risky" && c.Disposable:
		return "Please use a permanent address, not a throwaway one."
	}
	return ""
}

func signup(w http.ResponseWriter, r *http.Request) {
	email := strings.TrimSpace(r.PostFormValue("email"))
	if email == "" {
		http.Error(w, "Enter your email.", http.StatusBadRequest)
		return
	}
	if msg := problem(email, lookup(r.Context(), email)); msg != "" {
		http.Error(w, msg, http.StatusUnprocessableEntity)
		return
	}
	// Create the user here.
	w.WriteHeader(http.StatusCreated)
}

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