diff -r c9b7e14b7bb1 redmine34/app/models/mail_handler.rb
--- a/redmine34/app/models/mail_handler.rb	Tue Apr 30 13:33:29 2019 +0200
+++ b/redmine34/app/models/mail_handler.rb	Tue Apr 30 16:01:21 2019 +0200
@@ -159,7 +159,9 @@
   def dispatch
     headers = [email.in_reply_to, email.references].flatten.compact
     subject = email.subject.to_s
-    if headers.detect {|h| h.to_s =~ MESSAGE_ID_RE}
+    if issue = get_issue_from_receiver_addresses
+      receive_issue_reply(issue.id)
+    elsif headers.detect {|h| h.to_s =~ MESSAGE_ID_RE}
       klass, object_id = $1, $2.to_i
       method_name = "receive_#{klass}_reply"
       if self.class.private_instance_methods.collect(&:to_s).include?(method_name)
@@ -377,6 +379,25 @@
     keyword
   end
 
+  def get_issue_from_receiver_addresses
+    local, domain = Setting.mail_from.to_s.split("@")
+    return nil unless local && domain && Setting.plus_ticketid?
+    local = Regexp.escape(local)
+
+    [:to, :cc, :bcc].each do |field|
+      header = @email[field]
+      next if header.blank? || header.field.blank? || !header.field.respond_to?(:addrs)
+      header.field.addrs.each do |addr|
+        if addr.domain.to_s.casecmp(domain)==0 && addr.local.to_s =~ /\A#{local}\+([\d]+)\z/
+          if issue = Issue.find_by_id($1)
+            return issue
+          end
+        end
+      end
+    end
+    nil
+  end
+
   def get_project_from_receiver_addresses
     local, domain = handler_options[:project_from_subaddress].to_s.split("@")
     return nil unless local && domain
diff -r c9b7e14b7bb1 redmine34/app/models/mailer.rb
--- a/redmine34/app/models/mailer.rb	Tue Apr 30 13:33:29 2019 +0200
+++ b/redmine34/app/models/mailer.rb	Tue Apr 30 16:01:21 2019 +0200
@@ -441,13 +441,19 @@
   end
 
   def mail(headers={}, &block)
-    headers.reverse_merge! 'X-Mailer' => 'Redmine',
+    reply_to = nil
+    if Setting.plus_ticketid? && @issue && @issue.id?
+        reply_to = Setting.mail_from.to_s.sub('@', "+#{@issue.id}@")
+    end
+    headers.reverse_merge!({'X-Mailer' => 'Redmine',
             'X-Redmine-Host' => Setting.host_name,
             'X-Redmine-Site' => Setting.app_title,
             'X-Auto-Response-Suppress' => 'All',
             'Auto-Submitted' => 'auto-generated',
+            'Reply-To' => reply_to,
             'From' => Setting.mail_from,
             'List-Id' => "<#{Setting.mail_from.to_s.gsub('@', '.')}>"
+    }.reject{ |k,v| v.nil? })
 
     # Replaces users with their email addresses
     [:to, :cc, :bcc].each do |key|
diff -r c9b7e14b7bb1 redmine34/app/views/settings/_notifications.html.erb
--- a/redmine34/app/views/settings/_notifications.html.erb	Tue Apr 30 13:33:29 2019 +0200
+++ b/redmine34/app/views/settings/_notifications.html.erb	Tue Apr 30 16:01:21 2019 +0200
@@ -8,6 +8,8 @@
 
 <p><%= setting_check_box :plain_text_mail %></p>
 
+<p><%= setting_check_box :plus_ticketid %></p>
+
 <p><%= setting_select(:default_notification_option, User.valid_notification_options.collect {|o| [l(o.last), o.first.to_s]}) %></p>
 
 </div>
diff -r c9b7e14b7bb1 redmine34/config/locales/en.yml
--- a/redmine34/config/locales/en.yml	Tue Apr 30 13:33:29 2019 +0200
+++ b/redmine34/config/locales/en.yml	Tue Apr 30 16:01:21 2019 +0200
@@ -388,6 +388,7 @@
   setting_mail_from: Emission email address
   setting_bcc_recipients: Blind carbon copy recipients (bcc)
   setting_plain_text_mail: Plain text mail (no HTML)
+  setting_plus_ticketid: Append ticket id to sender address (Reply-To)
   setting_host_name: Host name and path
   setting_text_formatting: Text formatting
   setting_wiki_compression: Wiki history compression
diff -r c9b7e14b7bb1 redmine34/config/settings.yml
--- a/redmine34/config/settings.yml	Tue Apr 30 13:33:29 2019 +0200
+++ b/redmine34/config/settings.yml	Tue Apr 30 16:01:21 2019 +0200
@@ -84,6 +84,8 @@
   default: 1
 plain_text_mail:
   default: 0
+plus_ticketid:
+  default: 0
 text_formatting:
   default: textile
 cache_formatted_text: