Another idea: file the email in your reference material, and put a tickler reminder for whenever you think this might be appropriate (a month? two months?). When the time comes and the tickler comes up, if it's not time yet, you can just quickly re-file it to remind you again in the future. That's what I have done with things I know will be needed sometime, but not sure when. Then I get a monthly reminder, which is quickly dismissed if not relevant yet.