Ruby on rails 4 Rails如何使用mailgun沙盒从本地主机发送电子邮件

我是RoR新手，所以我不确定自己做错了什么。我不断地发现这个错误：

EmailUserRegistration#send_admin_user_registration: 
processed  outbound  mail in 1035.4ms

Completed 500 Internal Server Error in 1119ms (ActiveRecord: 1.1ms)

Mailgun::CommunicationError (SSL_connect returned=1 errno=0 state=SSLv3 read server certificate B: certificate verify failed):

app/mailers/email_user_registration.rb:11:in `send_admin_user_registration'
app/controllers/users_controller.rb:22:in `create'

这是我的ApplicationMailer：

class EmailUserRegistration < ApplicationMailer


 def send_admin_user_registration(user)
   @user = user
   mg_client = Mailgun::Client.new 'key-sandboxkey'
   message_params = {:from    => 'myemail@gmail.com',
                     :to      => 'myemail@gmail.com',
                     :subject => 'Sample Mail using Mailgun API',
                     :text    => 'This mail is sent using Mailgun API via mailgun-ruby'}
   mg_client.send_message 'https://api.mailgun.net/v3/sandbox1fc33f2b63734b8cb71c613f401dbbc1.mailgun.org', message_params
 end
end

结束

任何有助于这项工作将是伟大的！谢谢

def create
@user = User.new(user_params)
if @user.save

    EmailUserRegistration.send_admin_user_registration(@user).deliver

  session[:user_id] = @user.id
  redirect_to '/signup_success'
else
    render 'new'
end