sentences:  This is typically straightforward; a valid certificate must be obtained and STARTTLS must be enabled on the receiving organization's email server.To prevent downgrade attacks organizations can send their domain to the 'STARTTLS Policy List'
  
  Most full-featured email clients provide native support for S/MIME secure email (digital signing and message encryption using certificates).Other encryption options include PGP and GNU Privacy Guard (GnuPG).