Some people I know have dozens of mail folders, and a semi-organised way of putting their mail into them. I did so myself for many years. Let me tell you how a professional does it.
I was recently convinced by a Tom Limoncelli system administration book that using only a handful of mail folders was better than using many folders, and better than just keeping everything in the inbox.
His method is to put everything he keeps into a folder called save, except receipts, which go into receipts, and of course sent mail goes in the sent folder automatically.
Then he uses his inbox as a quick-sorting station where has five or six strategies. Each email he either deletes, or reads and deletes, or reads and saves, or reads and follows up and saves, or reads and follows up and deletes, or reads, makes a diary entry, and saves, or reads, makes a diary entry, and deletes.
Search my mail
When he needs to find a mail, he searches the save folder, or the receipts folder, or the sent folder.
Old mail is almost always easier to search than to browse.
Move my mail
Sometimes it’s useful to move mail from one computer to another, and from one type of mail client to another.
Usually this means the mail, which is kept as sets of files in folders on a computer, will need to be transformed into basically similar set of files in folders somewhere else, and during the process of achieving that transformation, by Murphy’s Law it is usually necessary to do some particular thing with every mail folder individually, and in this case, time sitting in front of a sluggish computer will multiply.
Delete my mail
By deleting most of your mail, you’ll keep it at bay, and it will be easier to move it if you need to. By keeping only a few mails, you’ll have a convenient set of commemorations.
Save my mail
If you are worried you might delete something and want it later, then consider a fourth folder, which automatically gets a copy of everything. It will be a big folder, but not so important that if you change providers or devices you can’t just delete it, and it will contain everything you ever received.
But Tom Limoncelli’s advice was succinct: E-mail is essentially ephemeral.