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.

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