Ask yourself if you will ever need to reference that email again. If the answer is no, then hit the delete button. The fewer emails you have floating around your system, the easier it is to keep an overview and the less disk space and system resources you use. Don't save emails just because you can. Make use of the cut and paste function.
Less clutter means more control.
If you want to keep every copy of a newsletter issue as a reference base, check first to see whether there are searchable online archives - you may not need to store them yourself. Modern email software allows you to filter incoming mail. If you let every email into your inbox, you might find that the sheer volume of mail results in important messages getting lost in the crowd. This means my inbox contains only personal messages, and I can browse my newsletter folder later when I have time to catch up. Filing emails into different folders is the final key to good email management.