Only return the address if incoming emails is enabled:

Feedback from:
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/3363#note_12669123

app/models/project.rb

@@ -529,9 +529,11 @@ class Project < ActiveRecord::Base
   end
   def new_issue_address(author)
+    if Gitlab::IncomingEmail.enabled?
       Gitlab::IncomingEmail.reply_address(
         "#{path_with_namespace}+#{author.authentication_token}")
     end
+  end
   def build_commit_note(commit)
     notes.new(commit_id: commit.id, noteable_type: 'Commit')

spec/models/project_spec.rb

@@ -130,13 +130,14 @@ describe Project, models: true do
   end
   describe "#new_issue_address" do
-    before do
-      stub_incoming_email_setting(address: "p+%{key}@gl.ab")
-    end
-
     let(:project) { create(:empty_project, path: "somewhere") }
     let(:user) { create(:user) }
+    context 'incoming email enabled' do
+      before do
+        stub_incoming_email_setting(enabled: true, address: "p+%{key}@gl.ab")
+      end
+
       it 'returns the address to create a new issue' do
         token = user.authentication_token
         address = "p+#{project.namespace.path}/#{project.path}+#{token}@gl.ab"
@@ -145,6 +146,17 @@ describe Project, models: true do
       end
     end
+    context 'incoming email disabled' do
+      before do
+        stub_incoming_email_setting(enabled: false)
+      end
+
+      it 'returns nil' do
+        expect(project.new_issue_address(user)).to be_nil
+      end
+    end
+  end
+
   describe 'last_activity methods' do
     let(:project) { create(:project) }
     let(:last_event) { double(created_at: Time.now) }