Storage may be cheap, but that's hardly an excuse for being cluttered.
Ask yourself: When are you ever going to read all those email again? When is *anybody* ever going to read them again. And the more you have, the less likely it is that they ever will be read, because the more you have, the more time it will take to go through them.
And don't tell me that doesn't matter because it's easy to run a search -- the same still applies, and you'd only bother running a search if you had something specific you wanted to search for. Is there anything in your 2003 email archives that you are likely to want to search for? The answer to this question may well be 'yes'; you know your archives better than I do; but I'll tell you this: if you haven't found the need to search an archive over the last five years, then the odds are diminishingly small that you'll need to in the future.
My advice is to keep your archives, but take the time to filter out the stuff you really don't need or want any more.
First, sort the list of emails by size.
This will give you all the ones with attachments. The odds are most of the big stuff can be deleted. Most of the stuff you want to keep you'll already have extracted from your email and saved somewhere else. So feel free to delete them. There will also be obsolete software, video and flash attachments that were funny five years ago, and other junk. Deleting all this stuff will free up a substantial portion of your disk usage.
Next sort the list by name of sender.
This might sound odd, but it's a very quick way to see who you were talking to all those years ago. There might be a few surprises in there. People you'd lost touch with an virtually forgotten about. Maybe this is your chance to remind yourself to get back in touch? If so, then the exercise has been worthwhile even if you don't delete anything. Or maybe you know you don't want to talk to them. In that case, you do really want to keep those old emails from them? Get rid of them. It's cathartic.
Next, check if you've been subscribed to any mailing lists over the years.
Possibly you'll want to keep some of those archives, but equally there can be a lot of pretty mundane chatter on these things, and the bits that are relevant are often only relevant for the moment. It depends a lot on the individual lists, but my experience is that content five years old or more is unlikely to still be of much value. And in any case, most good mailing lists have their own archives online. So your own copies in your archives may be pretty pointless. Be ruthless and delete them.
My guess is that if you followed that advice, your email archives are now about a quarter of their original size. And nothing of value was lost.
In fact, doing an exercise like this every now and then can actually be helpful. Not because it saves disk space, but because it means that you do actually go back every now and then and look at what you were doing a few years ago. It's remarkable the things you forget over time. Sometimes its good to be reminded. Other times you may not want to be reminded, but that's what the delete key is for; delete it, and you won't need to be reminded of it again when you do this same process next time.