I should have included this section of the bounce notification:

  SMTP error from remote mail server after initial connection:
    host mx1.comcast.net [68.87.26.147]: 554 imta23.westchester.pa.mail.comcast.netcomcast 74.115.253.198 Comcast block for spam.  Please seehttp://postmaster.comcast.net/smtp-error-codes.php#BL000000

Bob