diff --git a/app/controllers/categories_controller.rb b/app/controllers/categories_controller.rb
index 74219e7d995..9de2c23383e 100644
--- a/app/controllers/categories_controller.rb
+++ b/app/controllers/categories_controller.rb
@@ -87,6 +87,9 @@ class CategoriesController < ApplicationController
       if category_params.key? :email_in and category_params[:email_in].length == 0
         # properly null the value so the database constrain doesn't catch us
         category_params[:email_in] = nil
+      elsif category_params.key? :email_in and existing_category = Category.find_by(email_in: category_params[:email_in]) and existing_category.id != @category.id
+        # check if email_in address is already in use for other category
+        return render_json_error I18n.t('category.errors.email_in_already_exist', {email_in: category_params[:email_in], category_name: existing_category.name})
       end
 
       category_params.delete(:position)
diff --git a/config/locales/server.en.yml b/config/locales/server.en.yml
index 5a172035731..e69873aefd9 100644
--- a/config/locales/server.en.yml
+++ b/config/locales/server.en.yml
@@ -341,6 +341,7 @@ en:
       uncategorized_parent: "Uncategorized can't have a parent category"
       self_parent: "A subcategory's parent cannot be itself"
       depth: "You can't nest a subcategory under another"
+      email_in_already_exist: "Incoming email address '%{email_in}' is already in use for '%{category_name}' category."
     cannot_delete:
       uncategorized: "Can't delete Uncategorized"
       has_subcategories: "Can't delete this category because it has sub-categories."
diff --git a/spec/controllers/categories_controller_spec.rb b/spec/controllers/categories_controller_spec.rb
index 9a7d342ae62..0f1a4e93476 100644
--- a/spec/controllers/categories_controller_spec.rb
+++ b/spec/controllers/categories_controller_spec.rb
@@ -174,6 +174,14 @@ describe CategoriesController do
         end
       end
 
+      it "returns 422 if email_in address is already in use for other category" do
+        @other_category = Fabricate(:category, name: "Other", email_in: "mail@examle.com" )
+        xhr :put, :update, id: @category.id, name: "Email", email_in: "mail@examle.com", color: "ff0", text_color: "fff"
+
+        expect(response).not_to be_success
+        expect(response.code.to_i).to eq(422)
+      end
+
       describe "success" do
 
         it "updates the group correctly" do