Provide a Mail policy setting to add custom subject (prefix) in every mail

Currently there is no way for administrator to add a custom prefix in Subject of mail for selected users. This is possible only via customization of Mail template. So there is a feature request to provide the Mail Policy setting which can add a custom prefix in every subject so that Administrator can normalize their mails with standard subject prefix.

This selection should also allow if prefix has to be added to only external mails or internal mails or both type of mails.

e.g.- There are group of users which are sending mails to external clients and Administrator wants to prefix the subject "Acme Insurance: " to every outgoing mail which their users sent to external users.