Index: test/fixtures/mail_handler/subject_japanese_3.eml
===================================================================
--- test/fixtures/mail_handler/subject_japanese_3.eml	(nonexistent)
+++ test/fixtures/mail_handler/subject_japanese_3.eml	(working copy)
@@ -0,0 +1,9 @@
+From: John Smith <JSmith@somenet.foo>
+To: "redmine@somenet.foo" <redmine@somenet.foo>
+Subject: =?iso-2022-jp?B?GyRCLSEbKEIgGyRCNF0/dDt6JUYlOSVIGyhC?=
+Date: Mon, 27 Aug 2018 09:30:00 +0900
+Message-ID: <87C31D42249DD0489D1A1444E3232DD7019D6183@foo.bar>
+
+The subject contains a "CIRCLED DIGIT ONE" character (U+2460).
+It is undefined in ISO-2022-JP but defined in some vendor-extended
+variants define it.
Index: test/unit/mail_handler_test.rb
===================================================================
--- test/unit/mail_handler_test.rb	(revision 17471)
+++ test/unit/mail_handler_test.rb	(working copy)
@@ -739,6 +739,21 @@
     assert_equal ja, issue.subject
   end
 
+  def test_add_issue_with_iso_2022_jp_ms_subject
+    issue = submit_email(
+              'subject_japanese_3.eml',
+              :issue => {:project => 'ecookbook'}
+            )
+    assert_kind_of Issue, issue
+    # The original subject is "① 丸数字テスト".
+    # CIRCLED DIGIT ONE character is not defined in ISO-2022-JP but
+    # defined in some vendor-extended variants such as ISO-2022-JP-MS.
+    # The character should be replaced with the replacement character
+    # instead of breaking the whole subject.
+    assert_match /丸数字テスト/, issue.subject
+    byebug
+  end
+
   def test_should_ignore_emails_from_locked_users
     User.find(2).lock!