Can anyone help me interpret this message from the error log?  I don't know what the SenderFilterConfig list is.

The subscriber's e-mail account is active, as she e-mailed me from her account to ask why she is not receiving posted messages from the list.  My personal e-mails to the user are being delivered OK.

              Last error: 5.7.105 554 5.7.105 SenderFilterAgent; Sender
                            denied as sender's email address is on
                            SenderFilterConfig list