At my work, we use a plugin in Outlook (for those of us who use Windows) to classify emails. We can classify the email as "General Business", "Private", "Sensitive", "Public", etc. This inserts some text at the bottom of each email before sending.
It would be excellent to have a plugin which allows one to add classifications to emails and to select a default one to append to each message.
Perhaps add a classification button to the bar of format options, which when clicked opens a drop down of classifications. Somewhere in the settings one could associate rich text with a classification name. Then right before the email is sent, the classification is appended.