This is how I dealt with my Inbox of about 6000 messages.
I realized that I wanted to save many of them for potential reference (I'm a pack rat). But also, of course, many of them could be trashed. I started with email #1 (my email client numbers them, and also provides some powerful filtering features). If I wanted to save it, I created a reasonable folder. For example, if the email was from a brokerage account, I searched for all of them and moved them to their own folder. If an email was to be trashed, and it was likely that there were more from the same sender to trash (e.g., spam), I searched and then trashed them all in one fell swoop. I then moved on to the next message and repeated the process -- very much like the processing steps in GTD, but automated in being able to find and deal with all similar items at once.
Of course, there are the emails that need to be answered, and action items uncovered. So those go in an appropriate folder or list.
But most of your emails will be reference or trash. Moving all emails from a certain sender (or several senders) to one folder can ensure that any needed reference info is available, yet quickly reduce the size of your inbox -- much faster than reading through every single one. If you ever get the time and inclination to clean out the reference email folders, they'll be there.