Mail delivery failed: returning message to sender

[Mail Delivery System]

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:

  sender@shapetotonto.com
    The mail server could not deliver mail to sender@shapetotonto.com.  The account or domain may not exist, they may be blacklisted, or missing the proper dns entries.