Just as I keep my 10,000+ files organized with a structured folder system, I do the same for the emails I keep.
I’ve got a folder for each major client, one for my company, one labeled Family, and one labeled Gregg for personal emails.
I don’t have as many email folders as I would folders for files because I’ve found it handy to be able to see all my email folders at one time in Outlook. It makes filing emails in my inbox easier since I can drag any email to its folder without scrolling through the list.
I don’t use a lot of sub-folders because they are hard to easily drag emails into.
As a result I tend to have a lot more emails per folder than I would for files. But Outlook has basic search features and I can quickly sort by who sent an email, who received it, when, etc.
At the beginning of each year I use Outlook’s archive function to take the last year’s emails and put them in their own PST file. That keeps the number of emails in any folder to a slightly more manageable number. Plus each PST file is smaller for faster backups.