diff --git a/app/mailers/emails/groups.rb b/app/mailers/emails/groups.rb
index 1654fc55bca..39527b00e9f 100644
--- a/app/mailers/emails/groups.rb
+++ b/app/mailers/emails/groups.rb
@@ -4,7 +4,7 @@ module Emails
       @membership = UsersGroup.find(user_group_id)
       @group = @membership.group
       @target_url = group_url(@group)
-      mail(to: @membership.user.email,
+      mail(cc: @membership.user.email,
            subject: subject("Access to group was granted"))
     end
   end
diff --git a/app/mailers/emails/issues.rb b/app/mailers/emails/issues.rb
index a096df9dc0d..516e3da84c8 100644
--- a/app/mailers/emails/issues.rb
+++ b/app/mailers/emails/issues.rb
@@ -4,10 +4,10 @@ module Emails
       @issue = Issue.find(issue_id)
       @project = @issue.project
       @target_url = project_issue_url(@project, @issue)
-      set_message_id("issue_#{issue_id}")
-      mail(from: sender(@issue.author_id),
-           to: recipient(recipient_id),
-           subject: subject("#{@issue.title} (##{@issue.iid})"))
+      mail_new_thread(@issue,
+                      from: sender(@issue.author_id),
+                      cc: recipient(recipient_id),
+                      subject: subject("#{@issue.title} (##{@issue.iid})"))
     end
 
     def reassigned_issue_email(recipient_id, issue_id, previous_assignee_id, updated_by_user_id)
@@ -15,10 +15,10 @@ module Emails
       @previous_assignee = User.find_by(id: previous_assignee_id) if previous_assignee_id
       @project = @issue.project
       @target_url = project_issue_url(@project, @issue)
-      set_reference("issue_#{issue_id}")
-      mail(from: sender(updated_by_user_id),
-           to: recipient(recipient_id),
-           subject: subject("#{@issue.title} (##{@issue.iid})"))
+      mail_answer_thread(@issue,
+                         from: sender(updated_by_user_id),
+                         cc: recipient(recipient_id),
+                         subject: subject("#{@issue.title} (##{@issue.iid})"))
     end
 
     def closed_issue_email(recipient_id, issue_id, updated_by_user_id)
@@ -26,10 +26,10 @@ module Emails
       @project = @issue.project
       @updated_by = User.find updated_by_user_id
       @target_url = project_issue_url(@project, @issue)
-      set_reference("issue_#{issue_id}")
-      mail(from: sender(updated_by_user_id),
-           to: recipient(recipient_id),
-           subject: subject("#{@issue.title} (##{@issue.iid})"))
+      mail_answer_thread(@issue,
+                         from: sender(updated_by_user_id),
+                         cc: recipient(recipient_id),
+                         subject: subject("#{@issue.title} (##{@issue.iid})"))
     end
 
     def issue_status_changed_email(recipient_id, issue_id, status, updated_by_user_id)
@@ -38,10 +38,10 @@ module Emails
       @project = @issue.project
       @updated_by = User.find updated_by_user_id
       @target_url = project_issue_url(@project, @issue)
-      set_reference("issue_#{issue_id}")
-      mail(from: sender(updated_by_user_id),
-           to: recipient(recipient_id),
-           subject: subject("#{@issue.title} (##{@issue.iid})"))
+      mail_answer_thread(@issue,
+                         from: sender(updated_by_user_id),
+                         cc: recipient(recipient_id),
+                         subject: subject("#{@issue.title} (##{@issue.iid})"))
     end
   end
 end
diff --git a/app/mailers/emails/merge_requests.rb b/app/mailers/emails/merge_requests.rb
index ea5671c4502..c4ca3a3b69b 100644
--- a/app/mailers/emails/merge_requests.rb
+++ b/app/mailers/emails/merge_requests.rb
@@ -4,10 +4,10 @@ module Emails
       @merge_request = MergeRequest.find(merge_request_id)
       @project = @merge_request.project
       @target_url = project_merge_request_url(@project, @merge_request)
-      set_message_id("merge_request_#{merge_request_id}")
-      mail(from: sender(@merge_request.author_id),
-           to: recipient(recipient_id),
-           subject: subject("#{@merge_request.title} (##{@merge_request.iid})"))
+      mail_new_thread(@merge_request,
+                      from: sender(@merge_request.author_id),
+                      cc: recipient(recipient_id),
+                      subject: subject("#{@merge_request.title} (##{@merge_request.iid})"))
     end
 
     def reassigned_merge_request_email(recipient_id, merge_request_id, previous_assignee_id, updated_by_user_id)
@@ -15,10 +15,10 @@ module Emails
       @previous_assignee = User.find_by(id: previous_assignee_id) if previous_assignee_id
       @project = @merge_request.project
       @target_url = project_merge_request_url(@project, @merge_request)
-      set_reference("merge_request_#{merge_request_id}")
-      mail(from: sender(updated_by_user_id),
-           to: recipient(recipient_id),
-           subject: subject("#{@merge_request.title} (##{@merge_request.iid})"))
+      mail_answer_thread(@merge_request,
+                         from: sender(updated_by_user_id),
+                         cc: recipient(recipient_id),
+                         subject: subject("#{@merge_request.title} (##{@merge_request.iid})"))
     end
 
     def closed_merge_request_email(recipient_id, merge_request_id, updated_by_user_id)
@@ -26,20 +26,20 @@ module Emails
       @updated_by = User.find updated_by_user_id
       @project = @merge_request.project
       @target_url = project_merge_request_url(@project, @merge_request)
-      set_reference("merge_request_#{merge_request_id}")
-      mail(from: sender(updated_by_user_id),
-           to: recipient(recipient_id),
-           subject: subject("#{@merge_request.title} (##{@merge_request.iid})"))
+      mail_answer_thread(@merge_request,
+                         from: sender(updated_by_user_id),
+                         cc: recipient(recipient_id),
+                         subject: subject("#{@merge_request.title} (##{@merge_request.iid})"))
     end
 
     def merged_merge_request_email(recipient_id, merge_request_id, updated_by_user_id)
       @merge_request = MergeRequest.find(merge_request_id)
       @project = @merge_request.project
       @target_url = project_merge_request_url(@project, @merge_request)
-      set_reference("merge_request_#{merge_request_id}")
-      mail(from: sender(updated_by_user_id),
-           to: recipient(recipient_id),
-           subject: subject("#{@merge_request.title} (##{@merge_request.iid})"))
+      mail_answer_thread(@merge_request,
+                         from: sender(updated_by_user_id),
+                         cc: recipient(recipient_id),
+                         subject: subject("#{@merge_request.title} (##{@merge_request.iid})"))
     end
   end
 
diff --git a/app/mailers/emails/notes.rb b/app/mailers/emails/notes.rb
index 8d1f17b0f81..7848d34ab2b 100644
--- a/app/mailers/emails/notes.rb
+++ b/app/mailers/emails/notes.rb
@@ -5,9 +5,10 @@ module Emails
       @commit = @note.noteable
       @project = @note.project
       @target_url = project_commit_url(@project, @commit, anchor: "note_#{@note.id}")
-      mail(from: sender(@note.author_id),
-           to: recipient(recipient_id),
-           subject: subject("#{@commit.title} (#{@commit.short_id})"))
+      mail_answer_thread(@commit,
+                         from: sender(@note.author_id),
+                         cc: recipient(recipient_id),
+                         subject: subject("#{@commit.title} (#{@commit.short_id})"))
     end
 
     def note_issue_email(recipient_id, note_id)
@@ -15,10 +16,10 @@ module Emails
       @issue = @note.noteable
       @project = @note.project
       @target_url = project_issue_url(@project, @issue, anchor: "note_#{@note.id}")
-      set_reference("issue_#{@issue.id}")
-      mail(from: sender(@note.author_id),
-           to: recipient(recipient_id),
-           subject: subject("#{@issue.title} (##{@issue.iid})"))
+      mail_answer_thread(@issue,
+                         from: sender(@note.author_id),
+                         cc: recipient(recipient_id),
+                         subject: subject("#{@issue.title} (##{@issue.iid})"))
     end
 
     def note_merge_request_email(recipient_id, note_id)
@@ -26,10 +27,10 @@ module Emails
       @merge_request = @note.noteable
       @project = @note.project
       @target_url = project_merge_request_url(@project, @merge_request, anchor: "note_#{@note.id}")
-      set_reference("merge_request_#{@merge_request.id}")
-      mail(from: sender(@note.author_id),
-           to: recipient(recipient_id),
-           subject: subject("#{@merge_request.title} (##{@merge_request.iid})"))
+      mail_answer_thread(@merge_request,
+                         from: sender(@note.author_id),
+                         cc: recipient(recipient_id),
+                         subject: subject("#{@merge_request.title} (##{@merge_request.iid})"))
     end
 
     def note_wall_email(recipient_id, note_id)
@@ -37,7 +38,7 @@ module Emails
       @project = @note.project
       @target_url = project_wall_url(@note.project, anchor: "note_#{@note.id}")
       mail(from: sender(@note.author_id),
-           to: recipient(recipient_id),
+           cc: recipient(recipient_id),
            subject: subject("Note on wall"))
     end
   end
diff --git a/app/mailers/emails/projects.rb b/app/mailers/emails/projects.rb
index 9f99c11ea30..6017d9192ec 100644
--- a/app/mailers/emails/projects.rb
+++ b/app/mailers/emails/projects.rb
@@ -4,7 +4,7 @@ module Emails
       @users_project = UsersProject.find user_project_id
       @project = @users_project.project
       @target_url = project_url(@project)
-      mail(to: @users_project.user.email,
+      mail(cc: @users_project.user.email,
            subject: subject("Access to project was granted"))
     end
 
@@ -12,7 +12,7 @@ module Emails
       @user = User.find user_id
       @project = Project.find project_id
       @target_url = project_url(@project)
-      mail(to: @user.email,
+      mail(cc: @user.email,
            subject: subject("Project was moved"))
     end
 
@@ -30,7 +30,7 @@ module Emails
       end
 
       mail(from: sender(author_id),
-           to: recipient,
+           cc: recipient,
            subject: subject("New push to repository"))
     end
   end
diff --git a/app/mailers/notify.rb b/app/mailers/notify.rb
index 84a0da0129d..d65bead9cfe 100644
--- a/app/mailers/notify.rb
+++ b/app/mailers/notify.rb
@@ -1,4 +1,6 @@
 class Notify < ActionMailer::Base
+  include ActionDispatch::Routing::PolymorphicRoutes
+
   include Emails::Issues
   include Emails::MergeRequests
   include Emails::Notes
@@ -16,6 +18,7 @@ class Notify < ActionMailer::Base
   default_url_options[:script_name] = Gitlab.config.gitlab.relative_url_root
 
   default from: Proc.new { default_sender_address.format }
+  default to:   Proc.new { project_sender_address.format }
   default reply_to: "noreply@#{Gitlab.config.gitlab.host}"
 
   # Just send email with 2 seconds delay
@@ -32,6 +35,17 @@ class Notify < ActionMailer::Base
     address
   end
 
+  # The default email address to send emails to. Includes the project name if possible.
+  def project_sender_address
+    if @project
+      address = default_sender_address
+      address.display_name = @project.name_with_namespace
+      address
+    else
+      default_sender_address
+    end
+  end
+
   # Return an email address that displays the name of the sender.
   # Only the displayed name changes; the actual email address is always the same.
   def sender(sender_id)
@@ -53,14 +67,6 @@ class Notify < ActionMailer::Base
     end
   end
 
-  # Set the Message-ID header field
-  #
-  # local_part - The local part of the message ID
-  #
-  def set_message_id(local_part)
-    headers["Message-ID"] = "<#{local_part}@#{Gitlab.config.gitlab.host}>"
-  end
-
   # Set the References header field
   #
   # local_part - The local part of the referenced message ID
@@ -93,4 +99,48 @@ class Notify < ActionMailer::Base
     subject << extra.join(' | ') if extra.present?
     subject
   end
+
+  # Return a string suitable for inclusion in the 'Message-Id' mail header.
+  #
+  # The message-id is generated from the unique URL to a model object.
+  def message_id(model)
+    model_name = model.class.model_name.singular_route_key
+    "<#{model_name}_#{model.id}@#{Gitlab.config.gitlab.host}>"
+  end
+
+  # Send an email that starts a new conversation thread,
+  # with headers suitable for grouping by thread in email clients.
+  #
+  # See: mail_answer_thread
+  def mail_new_thread(model, headers = {}, &block)
+    raise ArgumentError, '"To:" header will be overwritten; use "Cc:" or "Bcc:"' unless headers[:to].nil?
+    headers[:to] = project_sender_address.format
+
+    headers['Message-ID'] = message_id(model)
+
+    mail(headers, &block)
+  end
+
+  # Send an email that responds to an existing conversation thread,
+  # with headers suitable for grouping by thread in email clients.
+  #
+  # For grouping emails by thread, email clients heuristics require the answers to:
+  #
+  #  * have a subject that begin by 'Re: '
+  #  * have a 'In-Reply-To' or 'References' header that references the original 'Message-ID'
+  #  * have stable 'From' and 'To' headers between messages of the same thread
+  #
+  def mail_answer_thread(model, headers = {}, &block)
+    raise ArgumentError, '"To:" header will be overwritten; use "Cc:" or "Bcc:"' unless headers[:to].nil?
+    headers[:to] = project_sender_address.format
+
+    headers['In-Reply-To'] = message_id(model)
+    headers['References'] = message_id(model)
+
+    if (headers[:subject])
+      headers[:subject].prepend('Re: ')
+    end
+
+    mail(headers, &block)
+  end
 end
diff --git a/spec/mailers/notify_spec.rb b/spec/mailers/notify_spec.rb
index 547268d44f0..32ba6c9b863 100644
--- a/spec/mailers/notify_spec.rb
+++ b/spec/mailers/notify_spec.rb
@@ -10,7 +10,7 @@ describe Notify do
 
   shared_examples 'a multiple recipients email' do
     it 'is sent to the given recipient' do
-      should deliver_to recipient.email
+      should cc_to recipient.email
     end
   end
 
@@ -22,6 +22,23 @@ describe Notify do
     end
   end
 
+  shared_examples 'an email starting a new thread' do |message_id_prefix|
+    it 'has a discussion identifier' do
+      should have_header 'Message-ID',  /<#{message_id_prefix}(.*)@#{Gitlab.config.gitlab.host}>/
+    end
+  end
+
+  shared_examples 'an answer to an existing thread' do |thread_id_prefix|
+    it 'has a subject that begins with Re: ' do
+      should have_subject /^Re: /
+    end
+
+    it 'has headers that reference an existing thread' do
+      should have_header 'References',  /<#{thread_id_prefix}(.*)@#{Gitlab.config.gitlab.host}>/
+      should have_header 'In-Reply-To', /<#{thread_id_prefix}(.*)@#{Gitlab.config.gitlab.host}>/
+    end
+  end
+
   describe 'for new users, the email' do
     let(:example_site_path) { root_path }
     let(:new_user) { create(:user, email: 'newguy@example.com', created_by_id: 1) }
@@ -141,7 +158,7 @@ describe Notify do
         end
 
         it 'is sent to the assignee' do
-          should deliver_to assignee.email
+          should cc_to assignee.email
         end
       end
 
@@ -153,6 +170,7 @@ describe Notify do
           subject { Notify.new_issue_email(issue.assignee_id, issue.id) }
 
           it_behaves_like 'an assignee email'
+          it_behaves_like 'an email starting a new thread', 'issue'
 
           it 'has the correct subject' do
             should have_subject /#{project.name} \| #{issue.title} \(##{issue.iid}\)/
@@ -161,10 +179,6 @@ describe Notify do
           it 'contains a link to the new issue' do
             should have_body_text /#{project_issue_path project, issue}/
           end
-
-          it 'has the correct message-id set' do
-            should have_header 'Message-ID', "<issue_#{issue.id}@#{Gitlab.config.gitlab.host}>"
-          end
         end
 
         describe 'that are new with a description' do
@@ -179,6 +193,7 @@ describe Notify do
           subject { Notify.reassigned_issue_email(recipient.id, issue.id, previous_assignee.id, current_user) }
 
           it_behaves_like 'a multiple recipients email'
+          it_behaves_like 'an answer to an existing thread', 'issue'
 
           it 'is sent as the author' do
             sender = subject.header[:from].addrs[0]
@@ -201,16 +216,14 @@ describe Notify do
           it 'contains a link to the issue' do
             should have_body_text /#{project_issue_path project, issue}/
           end
-
-          it 'has the correct reference set' do
-            should have_header 'References', "<issue_#{issue.id}@#{Gitlab.config.gitlab.host}>"
-          end
         end
 
         describe 'status changed' do
           let(:status) { 'closed' }
           subject { Notify.issue_status_changed_email(recipient.id, issue.id, status, current_user) }
 
+          it_behaves_like 'an answer to an existing thread', 'issue'
+
           it 'is sent as the author' do
             sender = subject.header[:from].addrs[0]
             sender.display_name.should eq(current_user.name)
@@ -232,10 +245,6 @@ describe Notify do
           it 'contains a link to the issue' do
             should have_body_text /#{project_issue_path project, issue}/
           end
-
-          it 'has the correct reference set' do
-            should have_header 'References', "<issue_#{issue.id}@#{Gitlab.config.gitlab.host}>"
-          end
         end
 
       end
@@ -249,6 +258,7 @@ describe Notify do
           subject { Notify.new_merge_request_email(merge_request.assignee_id, merge_request.id) }
 
           it_behaves_like 'an assignee email'
+          it_behaves_like 'an email starting a new thread', 'merge_request'
 
           it 'has the correct subject' do
             should have_subject /#{merge_request.title} \(##{merge_request.iid}\)/
@@ -283,6 +293,7 @@ describe Notify do
           subject { Notify.reassigned_merge_request_email(recipient.id, merge_request.id, previous_assignee.id, current_user.id) }
 
           it_behaves_like 'a multiple recipients email'
+          it_behaves_like 'an answer to an existing thread', 'merge_request'
 
           it 'is sent as the author' do
             sender = subject.header[:from].addrs[0]
@@ -311,6 +322,7 @@ describe Notify do
           subject { Notify.merged_merge_request_email(recipient.id, merge_request.id, merge_author.id) }
 
           it_behaves_like 'a multiple recipients email'
+          it_behaves_like 'an answer to an existing thread', 'merge_request'
 
           it 'is sent as the merge author' do
             sender = subject.header[:from].addrs[0]
@@ -329,10 +341,6 @@ describe Notify do
           it 'contains a link to the merge request' do
             should have_body_text /#{project_merge_request_path project, merge_request}/
           end
-
-          it 'has the correct reference set' do
-            should have_header 'References', "<merge_request_#{merge_request.id}@#{Gitlab.config.gitlab.host}>"
-          end
         end
       end
     end
@@ -394,7 +402,7 @@ describe Notify do
         end
 
         it 'is sent to the given recipient' do
-          should deliver_to recipient.email
+          should cc_to recipient.email
         end
 
         it 'contains the message from the note' do
@@ -426,6 +434,7 @@ describe Notify do
         subject { Notify.note_commit_email(recipient.id, note.id) }
 
         it_behaves_like 'a note email'
+        it_behaves_like 'an answer to an existing thread', 'commits'
 
         it 'has the correct subject' do
           should have_subject /#{commit.title} \(#{commit.short_id}\)/
@@ -444,6 +453,7 @@ describe Notify do
         subject { Notify.note_merge_request_email(recipient.id, note.id) }
 
         it_behaves_like 'a note email'
+        it_behaves_like 'an answer to an existing thread', 'merge_request'
 
         it 'has the correct subject' do
           should have_subject /#{merge_request.title} \(##{merge_request.iid}\)/
@@ -462,6 +472,7 @@ describe Notify do
         subject { Notify.note_issue_email(recipient.id, note.id) }
 
         it_behaves_like 'a note email'
+        it_behaves_like 'an answer to an existing thread', 'issue'
 
         it 'has the correct subject' do
           should have_subject /#{issue.title} \(##{issue.iid}\)/
@@ -538,7 +549,7 @@ describe Notify do
     end
 
     it 'is sent to recipient' do
-      should deliver_to 'devs@company.name'
+      should cc_to 'devs@company.name'
     end
 
     it 'has the correct subject' do
@@ -574,7 +585,7 @@ describe Notify do
     end
 
     it 'is sent to recipient' do
-      should deliver_to 'devs@company.name'
+      should cc_to 'devs@company.name'
     end
 
     it 'has the correct subject' do
diff --git a/spec/services/issues/close_service_spec.rb b/spec/services/issues/close_service_spec.rb
index d4f2cc1339b..7324650d534 100644
--- a/spec/services/issues/close_service_spec.rb
+++ b/spec/services/issues/close_service_spec.rb
@@ -22,7 +22,7 @@ describe Issues::CloseService do
 
       it 'should send email to user2 about assign of new issue' do
         email = ActionMailer::Base.deliveries.last
-        email.to.first.should == user2.email
+        email.cc.first.should == user2.email
         email.subject.should include(issue.title)
       end
 
diff --git a/spec/services/issues/update_service_spec.rb b/spec/services/issues/update_service_spec.rb
index 347560414e7..47d18565034 100644
--- a/spec/services/issues/update_service_spec.rb
+++ b/spec/services/issues/update_service_spec.rb
@@ -31,7 +31,7 @@ describe Issues::UpdateService do
 
       it 'should send email to user2 about assign of new issue' do
         email = ActionMailer::Base.deliveries.last
-        email.to.first.should == user2.email
+        email.cc.first.should == user2.email
         email.subject.should include(issue.title)
       end
 
diff --git a/spec/services/merge_requests/close_service_spec.rb b/spec/services/merge_requests/close_service_spec.rb
index a504f916b08..2c5a7c4f26c 100644
--- a/spec/services/merge_requests/close_service_spec.rb
+++ b/spec/services/merge_requests/close_service_spec.rb
@@ -22,7 +22,7 @@ describe MergeRequests::CloseService do
 
       it 'should send email to user2 about assign of new merge_request' do
         email = ActionMailer::Base.deliveries.last
-        email.to.first.should == user2.email
+        email.cc.first.should == user2.email
         email.subject.should include(merge_request.title)
       end
 
diff --git a/spec/services/merge_requests/update_service_spec.rb b/spec/services/merge_requests/update_service_spec.rb
index af5d3a3dc81..42ac05862b3 100644
--- a/spec/services/merge_requests/update_service_spec.rb
+++ b/spec/services/merge_requests/update_service_spec.rb
@@ -31,7 +31,7 @@ describe MergeRequests::UpdateService do
 
       it 'should send email to user2 about assign of new merge_request' do
         email = ActionMailer::Base.deliveries.last
-        email.to.first.should == user2.email
+        email.cc.first.should == user2.email
         email.subject.should include(merge_request.title)
       end