You can edit existing emails, although usually this would not be a good idea since you probably want to preserve the exact email you received. You can add something to the subject line without even choosing to edit it. Categories are fine for a preset list, but sometimes I like to make a comment and save it with the email to make it easier to find later. Sometimes the subject is blank or created programmatically with little relevance (like voicemails). So I add something to the subject, marking it with something identifiable so I can clearly see what was added. It was pointed out in the meeting that changing the subject line will break the default behavior for the built in search for “messages in this conversation” since it has to match the subject exactly (well, except for RE:, etc.). Keep this in mind if you use that feature. I usually just use the regular search feature, which just finds anything that includes that string.