Rails

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 Write the validator

app/validators/mailvet_validator.rb

require "json"
require "net/http"

class MailvetValidator < ActiveModel::EachValidator
  def validate_each(record, attribute, value)
    return if value.blank?

    check = lookup(value)
    return if check.nil? # a signup shouldn't die because of us

    hint = check["did_you_mean"]
    if hint.present?
      local = value.rpartition("@").first
      record.errors.add(attribute, "looks mistyped. Did you mean #{local}@#{hint}?")
    elsif check["verdict"] == "invalid"
      record.errors.add(attribute, "can't receive mail")
    elsif check["verdict"] == "risky" && check["disposable"]
      record.errors.add(attribute, "is a throwaway address. Please use a permanent one")
    end
  end

  private

  def lookup(email)
    uri = URI("https://mailvet.dev/v1/check")
    uri.query = URI.encode_www_form(email: email)
    headers = { "Authorization" => "Bearer #{ENV.fetch("MAILVET_KEY")}" }

    res = Net::HTTP.start(uri.host, uri.port, use_ssl: true,
                          open_timeout: 2, read_timeout: 5) do |http|
      http.get(uri.request_uri, headers)
    end
    return JSON.parse(res.body) if res.is_a?(Net::HTTPSuccess)

    Rails.logger.warn("mailvet: HTTP #{res.code}")
    nil
  rescue StandardError => e
    Rails.logger.warn("mailvet: no answer: #{e.message}")
    nil
  end
end

3 Add it to the model

app/models/user.rb

class User < ApplicationRecord
  validates :email, presence: true
  validates :email, mailvet: true, on: :create
end