Until recently messages to the mailing list from unsubscribed addresses were forwarded directly to the maillist administrator, and sender had no knowledge why message was delayed or dropped. Since the number of messages automatically forwarded to maillist administrator grew with time mostly because of spam, we have added extra sender validation on mail server level.
Unfortunately mail server checks envelope sender address and Mailman checks From: address. In most cases they match, but for example, google mail servers set envelope sender to google account address, while From: address may be any you like. In this case sender will receive non-delivery report, although address in From: is correct and subscribed to the mailing list.
As a workaround it is possible to subscribe second (envelope sender) address to the mailing list and disable delivery through Mailman web interface.