SMTP trust check

A message needs to include a proof of trust of all local recipients. This could be:

• The content is signed with a certificate issued by a trusted certification authority.
• The user has provided a private address that is on a white list. Bear in mind:
  • Local-part is case sensitive, as per RFC 5321.
  • If there is a Reply-To address, do not use the From address.

Note that the checking software may not support other protocols than application/pkcs7-signature.