[META] Authentication via "from" email

Problem/Motivation

Support for authentication via “from” email address. This can be done by providing a new Analyzer plugin for Inmail. We will provide basic authentication by only checking “from” header of an email. Beside this option, there will be more secure authentication option where an email has digitally signed and verified. However, that option will be optional. We will provide a new field on User entity (public key). By processing incoming emails we will extract the email address from the “from” header field and try to match it with an existing Drupal user. In case there is a user with this email and “require crypto sign” option was selected we will try to verify the the signature with the corresponding public key. This will be done by relying on GnuPGP PHP extension.