Index: app/models/mail_handler.rb
===================================================================
--- app/models/mail_handler.rb	(revision 3757)
+++ app/models/mail_handler.rb	(working copy)
@@ -49,10 +49,12 @@
       logger.info  "MailHandler: ignoring email from Redmine emission address [#{sender_email}]" if logger && logger.info
       return false
     end
-    @user = User.find_by_mail(sender_email)
-    if @user && !@user.active?
-      logger.info  "MailHandler: ignoring email from non-active user [#{@user.login}]" if logger && logger.info
-      return false
+    if !sender_email.empty?
+      @user = User.find_by_mail(sender_email)
+      if @user && !@user.active?
+        logger.info  "MailHandler: ignoring email from non-active user [#{@user.login}]" if logger && logger.info
+        return false
+      end
     end
     if @user.nil?
       # Email was submitted by an unknown user