Like the war on terror – though I don't have high hopes for this analogy making it beyond the opening sentence of today's column – the war on email is now about a decade old. Recently, I've noticed a hard-headedness creeping in: gone are the ingenious tricks for staying on top of it all, replaced by the frank acknowledgment that sometimes you can't. The website five.sentenc.es, for example, urges you to announce "a personal policy that all email responses... will be five sentences or less", and earlier this year Chris Anderson, organiser of the Ted conferences, proposed new rules for email, including the use of "NNTR" – "no need to respond" – as an act of generosity towards overburdened recipients. These are welcome developments, since the curse of most productivity advice is the assumption that, given the right techniques, you can fit it all in when, to be honest, who knows? But my unscientific surveys of email-deluged friends suggest this isn't the whole story. On the rare occasions I'm permitted to inspect these bursting inboxes, one thing's clear: half of what's in them shouldn't be there.

I'm not talking about spam. The real issue afflicting my friends – which I tend to berate them about, so maybe the problem will go away when they stop being my friends – is that they fail to observe a simple distinction. Every email that isn't garbage, I tell them as they back away in alarm, can be categorised as either "active" or "reference". And if you don't treat these differently, you're needlessly making things worse.

Allow me to explain. (Actually, I've locked the door – you've no choice.) This distinction, which owes much to the ubiquitous productivity expert David Allen, isn't just about email. For every single information-bearing thing in your life – every piece of paper, voicemail, Post-it – there's either a specific task associated with it, or it's something you're just keeping in case you need to consult it. An unpaid gas bill is active; a paid gas bill is reference, assuming you keep your gas bills. An unanswered email is active, unless you've decided not to answer; an answered email is reference, unless you're waiting in turn for a reply. (Waiting for something counts as an active task.) Even a photograph of your favourite aunt is one or the other. If you need to send a thank you note, it's active; if there's no outstanding task, it's reference, even if it's the kind of reference material you might affix to your fridge instead of filing away. Reference emails have no place in your inbox – they should be archived – yet people who complain of too much email frequently seem to have inboxes full of them, adding greatly to their sense of stress. Thus I propose an additional email rule: you don't get to complain about overload until the only things in your inbox are active. One tip: temporarily sort emails by sender; if any one person has sent you scores of messages, probably only the most recent few are active. Better yet, get into the habit of archiving reference emails as soon as they arrive, or as soon as they turn into reference emails through being answered.

Rigorously separating active stuff from reference stuff – not just in email, but everywhere at home and work – is remarkably liberating. Much of what you thought you had to deal with may not need dealing with at all. Finding things you need will be easier. And should we meet, I won't have to harangue you obnoxiously. Got it? NNTR.

oliver.burkeman@theguardian.com; twitter.com/oliverburkeman