Merge the places where exceptions could be raised

app/workers/email_receiver_worker.rb

@@ -24,6 +24,8 @@ class EmailReceiverWorker
     reason = nil
     case e
+    when Gitlab::Email::UnknownIncomingEmail
+      reason = "We couldn't figure out what the email is for."
     when Gitlab::Email::SentNotificationNotFoundError
       reason = "We couldn't figure out what the email is in reply to. Please create your comment through the web interface."
     when Gitlab::Email::ProjectNotFound

lib/gitlab/email/handler/create_issue_handler.rb

@@ -14,10 +14,11 @@ module Gitlab
         end
         def can_handle?
-          !!(project_namespace && project)
+          !!authentication_token
         end
         def execute
+          raise ProjectNotFound unless project
           validate_permission!(:create_issue)
           verify_record!(

lib/gitlab/email/handler/create_note_handler.rb

@@ -6,7 +6,8 @@ module Gitlab
     module Handler
       class CreateNoteHandler < BaseHandler
         def can_handle?
-          !!(mail_key && sent_notification)
+          # We want to raise SentNotificationNotFoundError for missing key
+          !!(mail_key.nil? || mail_key =~ /\A\w+\z/)
         end
         def execute

lib/gitlab/email/receiver.rb

@@ -16,6 +16,7 @@ module Gitlab
     class NoteableNotFoundError < ProcessingError; end
     class InvalidNoteError < ProcessingError; end
     class InvalidIssueError < ProcessingError; end
+    class UnknownIncomingEmail < ProcessingError; end
     class Receiver
       def initialize(raw)
@@ -30,11 +31,8 @@ module Gitlab
         if handler = Handler.for(mail, mail_key)
           handler.execute
-        elsif mail_key =~ %r{/|\+}
-          # Sent Notification mail_key would not have / or +
-          raise ProjectNotFound
         else
-          raise SentNotificationNotFoundError
+          raise UnknownIncomingEmail
         end
       end

spec/lib/gitlab/email/receiver_spec.rb

@@ -30,6 +30,14 @@ describe Gitlab::Email::Receiver, lib: true do
     )
   end
+  context "when we cannot find a capable handler" do
+    let(:email_raw) { fixture_file('emails/valid_reply.eml').gsub(mail_key, "!!!") }
+
+    it "raises a UnknownIncomingEmail" do
+      expect { receiver.execute }.to raise_error(Gitlab::Email::UnknownIncomingEmail)
+    end
+  end
+
   context "when the recipient address doesn't include a mail key" do
     let(:email_raw) { fixture_file('emails/valid_reply.eml').gsub(mail_key, "") }