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.