Greetings.

It may be related to authentication, but...
Authentication has not been part of (E)SMTP for a long time - an AUTH extension has been kicking around for a couple of years and sendmail has support for it starting from 8.10 (based on SASL). Other than that, some authentication has been provided through a hack called pop_before_smtp (similar to what you refer to).

The short story is:

1. You have to find out which method your site is using - $Mail::Sendmail::log may be helping you here;
2. I doubt very much that Mail::Sendmail has support for either ESMTP AUTH or POP_BEFORE_SMTP, and that means you may have to roll your own (might be hard);
3. Perhaps you can charm the postmaster to carve a hole in the relay rules, wide enough for your script to peep through - but don't count too much on it.

Cheers,
alf