Perl – How to make an email bot that replies to users not reply to auto-responses and get itself into mail loops


I have a bot that replies to users. But sometimes when my bot sends its reply, the user or their email provider will auto-respond (vacation message, bounce message, error from mailer-daemon, etc). That is then a new message from the user (so my bot thinks) that it in turn replies to. Mail loop!

I'd like my bot to only reply to real emails from real humans. I'm currently filtering out email that admits to being bulk precedence or from a mailing list or has the Auto-Submitted header equal to "auto-replied" or "auto-generated" (see code below). But I imagine there's a more comprehensive or standard way to deal with this. (I'm happy to see solutions in other languages besides Perl.)

NB: Remember to have your own bot declare that it is autoresponding! Include

Auto-Submitted: auto-reply

in the header of your bot's email.

My original code for avoiding mail loops follows. Only reply if realmail returns true.

sub realmail {
  my($email) = @_;
  $email =~ /\nSubject\:\s*([^\n]*)\n/s;
  my $subject = $1;
  $email  =~ /\nPrecedence\:\s*([^\n]*)\n/s;
  my $precedence = $1;
  $email  =~ /\nAuto-Submitted\:\s*([^\n]*)\n/s;
  my $autosub = $1;

  return !($precedence =~ /bulk|list|junk/i ||
           $autosub =~ /(auto\-replied|auto\-generated)/i ||
           $subject =~ /^undelivered mail returned to sender$/i

(The Subject check is surely unnecessary; I just added these checks one at a time as problems arose and the above now seems to work so I don't want to touch it unless there's something definitively better.)

Best Solution

RFC 3834 provides some guidance for what you should do, but here are some concrete guidelines:

Set your envelope sender to a different email address than your auto-responder so bounces don't feed back into the system.

I always store in a database a key of when an email response was sent from a specific address to another address. Under no circumstance will I ever respond to the same address more than once in a 10 minute period. This alone stopped all loops, but doesn't ensure nice behavior (auto-responses to mailing lists are annoying).

Make sure you add any permutation of header that other people are matching on to stop loops. Here's the list I use:

X-Loop: autoresponder
Auto-Submitted: auto-replied
Precedence: bulk (autoreply)

Here are some header regex's I use to avoid loops and to try to play nice:


I also avoid responding if any of these are the envelop senders:

if ($sender eq ""
    || $sender =~ /^(?:request|owner|admin|bounce|bounces)-|-(?:request|owner|admin|bounce|bounces)\@|^(?:mailer-daemon|postmaster|daemon|majordomo|ma
ilman|bounce)\@|(?:listserv|listsrv)/i) {
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