[Taclist] Mail delivery failed: returning message to sender

This message was created automatically by mail delivery software. A message that you sent could not be delivered to one or more of its recipients. This is a permanent error. The following address(es) failed: taclist@mailman.xmission.com host mailman.xmission.com [166.70.13.29] SMTP error from remote mail server after end of data: 550 header syntax