Email conundrum, might be MIME::Lite vs. Earthlink

[dws]

I'm managing the back-end of a conference website. People register on-line (https:, of course). The registration system collects and saves their info, and then sends an email to a distribution list. This triggers billing, facilities planning, etc., etc.

Here's the problem. Starting about 4-5 weeks ago, people on the distribution list who have earthlink accounts stopped receiving the registration notifications. I can email to the distribution list by hand (using Eudora or Oulook), and they get my emails, but the automatic notifications aren't getting through to the earthlink accounts (or to people who have their domains hosted by earthlink). This is driving me nuts, and I'm wondering if anyone has run into either this problem or something similar.

Details:

The notification software is driven by a familar snippet.

   $msg = new MIME::Lite(
        From => "Registration System <register\@example.com>",
        To => $REGISTRATION,
        Subject => "Conference Registration",
        Type => 'text/plain',
        Encoding => '7Bit',
        Data => $body
    );

    $msg->delete("X-Mailer");
    $msg->send_by_sendmail(Sendmail => "/usr/sbin/sendmail") or do {
        # log the error
    }

No errors are getting logged (no surprise, since some of the email is getting through).

$REGISTRATION holds a string of the form "notify\@example.com". My .procmailrc at example.com contains a standard redistribution rule, of the form

  :0c
  * ^TOnotify
  ! [e-mail] [e-mail] [e-mail]

Again, this all works smoothly, except for earthlink. I'm not getting any bounces. But everyone gets the message when I mail to the distribution list from a standard client. Until I get this resolved, I'm forced to manually forward the notifications that I get back to the notification list.

It suspect that Earthlink has added some anti-spam facility that MIME::Lite (or my use of it) is running afoul of. But the folks with Earthlink accounts, one of whom is very tech savvy, can't find any info on their end.

I'd like some advice on how to adjust what I'm doing, or, failing that, advice on what kind of experiments I should try. Or pointers to more info. Extra credit given to a pointer to a MIME::Lite script that is known to work with Earthlink now. Thanks.

(If you know something, but don't want to go public with some technique that a spammer might use, /msg me.)