Trouble with mailing list

[Ignacio Burgueño <ignaciob@...>]

Excuse me if this is a problem on my end, but I recently noticed that
whenever I reply to a message on this list, the message gets delivered
correctly but shortly after that, I get a mail from
lua.to.typisch@xoxy.net with the subject "Undelivered Mail Returned to
Sender".

Part of the message follows:

~~~~~~~~~~~~~~~~~~~~
This is the mail system at host mx1.nonlin.de.

I'm sorry to have to inform you that your message could not
be delivered to one or more recipients. It's attached below.

For further assistance, please send mail to postmaster.

If you do so, please include this problem report. You can
delete your own text from the attached returned message.

                  The mail system

<+public/amavis@nonlin.de>: host mx1.nonlin.de[/var/run/cyrus/socket/lmtp]
   said: 550-Mailbox unknown.  Either there is no mailbox associated with this
   550-name or you do not have authorization to see it. 550 5.1.1 User unknown
   (in reply to RCPT TO command)

Final-Recipient: rfc822; +public/amavis@nonlin.de
Original-Recipient: rfc822;+public/amavis@nonlin.de
Action: failed
Status: 5.1.1
Remote-MTA: dns; mx1.nonlin.de
Diagnostic-Code: smtp; 550-Mailbox unknown.  Either there is no mailbox
   associated with this 550-name or you do not have authorization to see it.
   550 5.1.1 User unknown

~~~~~~~~~~~~~~~~~~~~