From Opera Software:
Opera Mail is a lightweight and customizable mail client for your Windows PC. It has many features which will make your every day email experience in a new way. View multiple emails with elegant tabs, threads help you smartly organize your mail. Keep up with the context of any mail thread, and quickly view previous messages in the conversation, and labels allow you to sort your messages quickly. It also has rule options to filter your mails the way you want. Opera Mail's built-in feed reader provides automatic notification of updates to your favorite websites.