Index: app/models/mail_handler.rb
===================================================================
--- app/models/mail_handler.rb  (revision 2659)
+++ app/models/mail_handler.rb  (working copy)
@@ -40,11 +40,11 @@
   # Processes incoming emails
   def receive(email)
     @email = email
-    @user = User.active.find_by_mail(email.from.to_a.first.to_s.strip)
+    find_user_from_email_header(email['return-path']) || find_user_from_email_header(email['sender']) || find_user_from_email_header(email['from'])
     unless @user
       # Unknown user => the email is ignored
       # TODO: ability to create the user's account
-      logger.info "MailHandler: email submitted by unknown user [#{email.from.first}]" if logger && logger.info
+      logger.info "MailHandler: email submitted by unknown user, Return-Path: #{email.header_string('return-path')}, Sender: #{email.header_string('sender')}, From: #{email.header_string('from')}" if logger && logger.info
       return false
     end
     User.current = @user
@@ -53,6 +53,22 @@

   private

+  def find_user_from_email_header(address_header)
+    address_spec = first_address_spec(address_header)
+    @user = User.active.find_by_mail(address_spec.strip) if address_spec
+  end
+
+  def first_address_spec(address_header)
+    return nil unless address_header && address_header.addrs
+    list = address_header.addrs.map {|addr|
+        if addr.address_group?
+        then addr.map {|a| a.spec }
+        else addr.spec
+        end
+    }.flatten
+    list.first
+  end
+
   MESSAGE_ID_RE = %r{^<redmine\.([a-z0-9_]+)\-(\d+)\.\d+@}
   ISSUE_REPLY_SUBJECT_RE = %r{\[[^\]]+#(\d+)\]}
   MESSAGE_REPLY_SUBJECT_RE = %r{\[[^\]]+msg(\d+)\]}