Windows Mail client that can store e-mails in eml files, one for each e-mail

I use a automatic file backup that search for updated and new files to make a mirror, it works flawless. The problem is I want to improve the way it backups mail. Using my actual mail client if I recive or delete one mail in a folder the backup program has to copy the entire 900 mb (or whatever) monolitic file again. I think the best solution is to store each mail in a separate file, I use this in mail severs, It has lots of advantages, not only backup, but I can not find a good solution for windows.
¿Is there any mail client that can use this kind of storage format, so I can continue to backup my e-mails without human intervenion and more eficiently?
I dont want to manualy export with human intervention, doubling the data space needed and using human intervention (both of wich produces errors), and resulting in no real time/on demand backup.