I replaced Microsoft Outlook and Outlook Express years ago with Thunderbird and have not looked back. It handles all of my email accounts (10+) without any slow down. It even handles my main RSS feeds and makes a good RSS news feed reader. I like the fact it is secure, cuts down on junk mail, has good phishing protection and constantly updated. It even has the best message search feature along with message tagging. And you can even add on tons of extensions and themes, like ThunderBrowse and Lightning (scheduler / organizer). I have not found anything better.