app/models/concerns/email_validatable.rb

@@ -0,0 +1,15 @@
+require 'mail'
+
+module EmailValidatable
+  extend ActiveSupport::Concern
+
+  class EmailValidator < ActiveModel::EachValidator
+    def validate_each(record, attribute, value)
+      begin
+        a = Mail::Address.new(value)
+      rescue Mail::Field::ParseError
+        record.errors[attribute] << (options[:message] || "is not a valid address")
+      end
+    end
+  end
+end

app/models/user.rb

@@ -1,10 +1,13 @@
 class User < ApplicationRecord
+  include EmailValidatable
+
   # Relations
   has_many :invitations, dependent: :destroy
 
   validates_uniqueness_of :cn
-  validates_uniqueness_of :email
   validates_length_of :cn, :minimum => 3
+  validates_uniqueness_of :email
+  validates :email, email: true
 
   # Include default devise modules. Others available are:
   # :confirmable, :lockable, :timeoutable, :trackable and :omniauthable