Brion Vibber brion@wikimedia.org wrote some months ago:
If "reject" from the mailing list means the sender gets a 5xx error during his SMTP session that is trying to deliver the message to "us", then "reject" is the better action.
If "reject" from the mailing list means that the sender's SMTP session succeeds (possibly before the mailing list manager even sees the message) and later a bounce message is sent to the purported sender, then "discard" is the better action for exactly the reason brion states.
If I had to guess, I'd guess the latter case is the situation for this list because I trust the Wikimedia employees know what they're doing.
:) Correct.
SMTP delivery succeeds. The message then gets passed into Mailman, who decides "oh I don't really want this" and sends back a "Dear so-and-so you're not allowed to post to this list, here's a copy of the spam message that was sent with your spoofed address" to fill up random peoples' inboxes.
Thus... we turn off rejection to save you spam debris and to save our servers from having to send out the spam debris.
If we could have it only send "sorry" mails on non-spam mails, that probably would be nice. Hopefully some day we can get there. :)
As spam is the current topic of another thread, I'll warm that up once again.
As I had mentioned, e. g. KDE balances spam debris vs. usability in a different way and they survive socially as well as technically.
But, to be more productive: If Wikimedia mailing lists were set up so that mails from non-members would need (si- lent) moderators' approval, I'd volunteer for those queues.
Tim