What's an Implicit Destination according to Mailman?

Email has explicit destinations in it, which are listed in the To: and Cc: headers.

However, email can also get implicit destinations set, which are carried around by Mail Transfer Agents (MTAs - things like sendmail, qmail, exim, MSExchange), and never make it into the normal headers of the email (except for appearing in Received: lines).

This is generally what happens if someone sends a Bcc:ed email. Also some Mail User Agents (MUAs - things like pine, mutt, elm, Eudora, etc) allow people to "bounce" a received email, which means it keeps all its explicit destinations, and gets sent to an MTA for delivery to a different mailbox.

All the above means that email can arrive at a mailbox (eg, thorfinn@tertius.net.au, or list-owners@queer.org.au) without having any explicit destination header set to that mailbox. That means it's delivered with an "implicit" destination.

Now... why does mailman (and other mailing list software) stop email with implicit destinations? Generally, spam arrives with implicit destinations, because spammers usually "relay" their mail via misconfigured MTAs, stealing the resources of that server to deliver thousands of copies of the email implicitly.