diff --git a/app/models/mailer.rb b/app/models/mailer.rb
--- a/app/models/mailer.rb
+++ b/app/models/mailer.rb
@@ -34,7 +34,6 @@ class Mailer < ActionMailer::Base
                     'Issue-Author' => issue.author.login
     redmine_headers 'Issue-Assignee' => issue.assigned_to.login if issue.assigned_to
     message_id issue
-    references issue
     @author = issue.author
     @issue = issue
     @users = to_users + cc_users
@@ -398,10 +397,11 @@ class Mailer < ActionMailer::Base
     end
 
     if @message_id_object
-      headers[:message_id] = "<#{self.class.message_id_for(@message_id_object)}>"
+      headers[:message_id] = "<#{self.class.message_id_for(@message_id_object, @users, @issue)}>"
     end
     if @references_objects
-      headers[:references] = @references_objects.collect {|o| "<#{self.class.references_for(o)}>"}.join(' ')
+      headers[:references] =
+        @references_objects.collect {|o| "<#{self.class.references_for(o, @users, @issue)}>"}.join(' ')
     end
 
     m = if block_given?
@@ -454,15 +454,18 @@ class Mailer < ActionMailer::Base
     h.each { |k,v| headers["X-Redmine-#{k}"] = v.to_s }
   end
 
-  def self.token_for(object, rand=true)
+  def self.token_for(object, users, issue)
     timestamp = object.send(object.respond_to?(:created_on) ? :created_on : :updated_on)
     hash = [
       "redmine",
       "#{object.class.name.demodulize.underscore}-#{object.id}",
       timestamp.strftime("%Y%m%d%H%M%S")
     ]
-    if rand
-      hash << Redmine::Utils.random_hex(8)
+    if users && users.present? && issue
+      ids = issue.visible_custom_field_values(users.first).map(&:custom_field_id).sort
+      if ids.present?
+        hash << Digest::SHA1.hexdigest(ids.join(","))[0, 8]
+      end
     end
     host = Setting.mail_from.to_s.strip.gsub(%r{^.*@|>}, '')
     host = "#{::Socket.gethostname}.redmine" if host.empty?
@@ -470,14 +473,14 @@ class Mailer < ActionMailer::Base
   end
 
   # Returns a Message-Id for the given object
-  def self.message_id_for(object)
-    token_for(object, true)
+  def self.message_id_for(object, users, issue)
+    token_for(object, users, issue)
   end
 
   # Returns a uniq token for a given object referenced by all notifications
   # related to this object
-  def self.references_for(object)
-    token_for(object, false)
+  def self.references_for(object, users, issue)
+    token_for(object, users, issue)
   end
 
   def message_id(object)
diff --git a/test/unit/mailer_test.rb b/test/unit/mailer_test.rb
--- a/test/unit/mailer_test.rb
+++ b/test/unit/mailer_test.rb
@@ -245,7 +245,6 @@ class MailerTest < ActiveSupport::TestCa
     Mailer.deliver_issue_add(issue)
     mail = last_email
     assert_match /^redmine\.issue-2\.20060719190421\.[a-f0-9]+@example\.net/, mail.message_id
-    assert_include "redmine.issue-2.20060719190421@example.net", mail.references
   end
 
   def test_issue_edit_message_id
@@ -255,7 +254,7 @@ class MailerTest < ActiveSupport::TestCa
     Mailer.deliver_issue_edit(journal)
     mail = last_email
     assert_match /^redmine\.journal-3\.\d+\.[a-f0-9]+@example\.net/, mail.message_id
-    assert_include "redmine.issue-2.20060719190421@example.net", mail.references
+    assert_match /^redmine\.issue-2\.20060719190421\.[a-f0-9]+@example\.net/, mail.references
     assert_select_email do
       # link to the update
       assert_select "a[href=?]",
@@ -267,7 +266,7 @@ class MailerTest < ActiveSupport::TestCa
     message = Message.find(1)
     Mailer.message_posted(message).deliver
     mail = last_email
-    assert_match /^redmine\.message-1\.\d+\.[a-f0-9]+@example\.net/, mail.message_id
+    assert_include "redmine.message-1.20070512151532@example.net", mail.message_id
     assert_include "redmine.message-1.20070512151532@example.net", mail.references
     assert_select_email do
       # link to the message
@@ -281,7 +280,7 @@ class MailerTest < ActiveSupport::TestCa
     message = Message.find(3)
     Mailer.message_posted(message).deliver
     mail = last_email
-    assert_match /^redmine\.message-3\.\d+\.[a-f0-9]+@example\.net/, mail.message_id
+    assert_include "redmine.message-3.20070512151802@example.net", mail.message_id
     assert_include "redmine.message-1.20070512151532@example.net", mail.references
     assert_select_email do
       # link to the reply
@@ -652,7 +651,8 @@ class MailerTest < ActiveSupport::TestCa
 
   def test_token_for_should_strip_trailing_gt_from_address_with_full_name
     with_settings :mail_from => "Redmine Mailer<no-reply@redmine.org>" do
-      assert_match /\Aredmine.issue-\d+\.\d+\.[0-9a-f]+@redmine.org\z/, Mailer.token_for(Issue.generate!)
+      assert_match /\Aredmine.issue-\d+\.\d+\.[0-9a-f]+@redmine.org\z/,
+                   Mailer.token_for(Issue.generate!, [User.find(1)], Issue.find(1))
     end
   end