You can certainly backup emails, and we always recommend that be done. In particular in your case with such a wealth of data and information you have stored in them.
Microsoft explains how to backup emails here;
But if you then delete those emails from within OE, you won't be able to access them from the backup. And I would add that if you do delete them, then those backups cease to be backups, but become your only copy.
However, there may be a way to do this. I found a link in Google when I searched for Outlook Express How to relocate email folders, and I found this Is there any way to get my Outlook Express email to save on the "D" drive?. Now, that talks about moving the stored email folders to another partition, or another internal hard drive, but I wonder if it would also work with an external drive.
I would be very cautious about this though. If a mistake is made you could lose all those emails, so I would ensure you have a backup already, preferably two backups, on two separate storage media, and do not use the external you intend to use for this experiment.
I hope that helps, and good luck.
My computer is installed with XP and I use Outlook Express for my e-mails, however, I have a hugh number of e-mails relating to my Family History research that I wish to retain. Can I move them from my computer to an external hard drive to free up space on my pc and still be able to access and read the messages on the hard drive, can this be done?