This is a postfix/sendmail milter that determines/blocks the system user for local SMTP connections.
Basically, when you manage a shared webserver you may allow your users to send mail from their web sites. If you use something like mod_ruid2 you can make sure the user's web sites run under their own user account. Building on this I figured it should be possible to figure out who sent an e-mail from such a server, or more specifically in case of problems I'd like to find out which customer to blame.
With this little script added to my postfix setups every message that's sent has a header added that indicates who sent it. I can even modify the script to blacklist certain users from sending mail, which is probably more customer-friendly than just disabling their entire web site.
More information about this on the milter-sysuser Github page.