The Desktop Email Client is an application that is installed on your local machine that fetches emails, address books, calendar entries etc from your accounts across several services. Having a few email accounts doesn't really call for using a Desktop Email Client, but when you have to manage several email accounts, then having the mail conveniently located at one easy to access location on your desktop, can significantly reduce the time you would spend accessing these services individually.
The most popular Desktop Email Client (Especially for Windows) is Microsoft Outlook. There are however, other Desktop Email Clients that are (unlike Microsoft Outlook) free.
Here's a small list of some Desktop Email Clients that are alternatives to Microsoft Outlook. Taking the time to find one that works, and then narrowing down that search to find one that works for you, will most definitely help you reduce the average time you spend reading and reverting emails.
Mozilla Thunderbird : (Windows, Mac and Linux) – Thunderbird with the Lightning extension is a Desktop Email Client just like Microsoft Outlook. The primary advantage being that Mozilla Thunderbird is FREE. So if you want the closest possible alternative to Microsoft Outlook, then go ahead and download Mozilla Thunderbird.
Windows Live Mail Desktop: (Windows only) – If you don’t want to purchase Microsoft office but you are still keen on Windows, then Windows Live Mail is the Desktop Email Client for you. Without having to buy any premium membership, you can access your Hotmail or Live emails. If this is what you were looking for, download Windows Live Mail Desktop.
Evolution: (Windows and Linux) – The Default Desktop Email Client for those using Linux. Its looks just the same as Microsoft Outlook, however the Windows version of this Desktop Email Client isn't very well developed. There are already quite a few players in the Desktop Email Client space for Windows so probably having this on windows wouldn't change much.
Zimbra: (Windows, Mac and Linux) – Zimbra, a product of Yahoo, is a relatively new player in the Desktop Email Client market, and it comes with features that can make you want to replace Mozilla Thunderbird or Evolution, at least on the Linux machine. This Email Client allows you to access your Yahoo email on your desktop, without having to purchase a premium membership. If you like how that sounds, download the Zimbra Desktop Email Client
As of right now, these are the biggest players in the Desktop Email Client sphere, There are more web based clients out there. Shall have a separate post about those as well very soon.