If you have ever wanted to know which are the most popular email clients that people worldwide are using to read /open their emails, take a look at this graph.

Microsoft Outlook (including Outlook Express) is the most popular desktop email software followed by web-based email services from Yahoo Mail and Windows Live Hotmail.

These stats are based on the email client usage of more than 300 million people that was done by Campaign Monitor for some six months.

The company embeds a tracking image into their email messages and uses the referral information to determine the email client that was used to open that particular email.

That means if a person is reading his emails in offline mode or has turned off automatic image downloads (like in Gmail) or is using an email client that cannot display images (like older BlackBerry phones), his email client will go undetected.

If you are reading this story on a mobile phone with images turned off, here’s a text version of the same graph:

Email client

Popularity

Microsoft Outlook

39.63%

Yahoo! Mail

15.65%

Hotmail

15.35%

Apple Mail

8.25%

iPhone / iPod Touch

5.78%

Gmail

5.51%

AOL Mail

2.76%

Lotus Notes 6

1.72%

Thunderbird 2

1.12%

Others

4.23%

It is also important to note that the above graph represents popularity of email clients and not email services. For instance, one could be using POP or IMAP to read his Gmail messages inside Outlook so that will increase the share of Outlook and not that of Gmail.

There’s little doubt that Yahoo! Mail is the more popular email service out there but another reason why its ranking so high in the list of email clients is because the free version of Yahoo Mail doesn’t offer POP functionality and therefore most people are using web browsers to check emails.