This bug was reported to the dev list on 5th June 2006: https://lists.webarch.co.uk/pipermail/mkdoc-dev/2006-June/000327.html
One MKDoc server keeps getting listed on the CBL email spam list, http://cbl.abuseat.org/
This is down to the use of Mail::CheckUser in flo/plugin/Account/Subscribe.pm — this is what the CBL says:
The Perl CheckUser module defaults to improper “HELO” and “MAIL FROM” strings: “localhost.localdomain” and “check at user.com” respectively. The former is illegal, the latter impersonates user.com – they probably don't like that. [Besides, by not using your own domain, some spam filters will lie to your RCPT TO.]
You will need to change $Helo_Domain = to be “<DNS name of your server>” and change $Sender_Addr to be something in your domain (eg: “check@<mydomain>”)
If you run ethereal and capture the helo MKDoc does indeed use the default of localhost.localdomain and the default email address of check at user.com.
Subscribe.pm needs to be fixed so that it uses the MKDoc public domain for the helo and the admin email address for the check.