I do use Gmail's labels but they are mostly for broader categories. If you have an email you'd like to reply to about, let's say, a payment system, you could ta

I do use Gmail's labels but they are mostly for broader categories. If you have an email you'd like to reply to about, let's say, a payment system, you could tag it "follow up about paypal micro-payments". This would be labeled as "work" or "finace" etc. but if you're getting back to it a few days or weeks later, you could view your cloud for paypal or micro-payments.