I am desperate for something in Outlook 2013 that allows arbitrary keyword tags to be added to every email, and then searched for later. I'm about to abandon it as an email client if I can't find one. If I had time I'd write it myself. Does anyone know of anything like this?
SimplyTag works well for me. It supports Outlook 2013
You can use outlook categories.
Categories in Outlook 2010 allow you to manage items in many different ways. There are three main types of categories that we recommend creating:
Project (can include people)
Location or activity
This question is tagged outlook-2010, if you were using Outlook 2010, then this third party tool could tag outlook emails. http://www.taglocity.com/downloads.html But if you are only after 2013 then it doesn't mention support for that version on their website.