Summary: The 'mailto:' or default mail program is a concept maintained by Windows and changed using Internet Explorer. Usually. We examine how to change the default mail program to a couple of popular services.
When viewing some web sites, I want to send them an email for whatever reason. I click, "contact us", a window opens and I type my question. When I press, "send", I realize that the email is being sent using Outlook, and as a result I asked to setup a POP address etc. I prefer to send and receive my messages using my Yahoo account. Can you tell me how I can set it up so that when sending a message as explained above, yahoo will come up as my email carrier?
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It's not that hard to change the system default mail program to one of a pre-defined list of applications. Not surprisingly, HotMail and Outlook Express are among the default choices.
If you want something else, like GMail or Yahoo Mail, things get a little more interesting.
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First the easy part.
The system has a notion of a "default" email client. That means this is the email program that will be used when a generic request to deal with email is received. The most common type of request is the HTML "mailto:" action you'll see on web pages. If a link is encoded with "mailto:" instead of "http:" when someone clicks on it the browser will fire up the default mail client for that user to compose a new email message.
In your case your system's default email client is obviously Outlook.
To change it, fire up Internet Explorer, hit the Tools menu, the Internet Options... menu item, and then click on the Programs tab. You should see something like this:

If you then click on the E-mail: dropdown, you'll see the full list of available options:

Simply select the email program you want to be the default program, and you're all set.
It's that simple, right?
Nope.
I can hear several of you yelling at your computers right now: "Why doesn't my list show Yahoo Mail like in the example!?".
Well, the "problem" is that something needs to be installed or set up on your system in order to make those options available. You'll notice that Thunderbird is on my list as well, and that showed up after I installed Thunderbird.
Yahoo was added to my list after I installed Yahoo Instant Messenger. Installing YIM actually has the potential to install quite a bit, so I definitely advise selecting the "custom" install and paying close attention to all the install options. I turned many things off, but left "Browser Services" checked, which I believe includes the feature we want:

But what about GMail?
GMail actually doesn't show up on my list, and yet it is my current default mail program. To do this, I installed the GMail Notifier. It's a handy little utility that notifies you when you have new GMail messages. And as I found out, one of the configuration options during the installation is to set GMail as the default mail handler:

That option is available again in the Notifier when it's running, so you can reset it properly should it ever get changed.
Now, as to why it doesn't show up in Internet Explorer's options list - I have no idea.
Related:
Ask Leo! - Are free email services worth it?
Ask Leo! - Why does IE not allow the selection of a default mail handler?
Article C2809 - October 13, 2006
in order to get yahoo mail in the drop down box in ie7 you must down load latest ver.of yahoo tool bar. when down load starts check make yahoo my default e-mail..this WORKS!! i just done this my self
Posted by: brian kimbrell at March 3, 2009 5:20 PMI tried loading yahoo instant messenger on 2 different computers to fix the above problem with getting my computer to default to Yahoo mail and it hasn't worked, doesn't show up in the menu. IE 7 is loaded and still can't figure out how to change it from Outlook.
Posted by: Betty Ann at April 11, 2009 1:59 PMThis did not work for me, even after installing Yahoo Messenger 9. I still don't have a menu item in that Mail options list. It only includes the three Microsoft products, Outlook, Outlook Express and Windows Live Hotmail. Isn't there a Registry change that can be performed to fix this?
Posted by: Jim H. at April 23, 2009 10:47 AMThe drop down list on Internet Options only contains three options on my IE 8 ( I followed your instructions ) Therefore I have no alternative but to go round the houses and send ( forward ) the whole e-mail aritcle . Is this because this is a version of IE ? Or is there something else that a non-computer expert needs to do or know ?
Posted by: Jyotish at April 30, 2009 2:01 AMHere's the problem. In trying to change my "mailto" it says I need to download yahoo messenger to make it an option in my drop down on the tools tab u sent me to. I HAVE YAHOO MESSENGER yet it still does not show up as an option under e-mail in the programs tab of internet options. So NOW what do I do to get yahoo as an option ????????????????????????????
Posted by: Keith Gonser at May 1, 2009 11:14 PMI have yahoo already I even deleted it and yet STILL yahoo will not aooear in the e-mail options of programs in my PROPERTIES of Internet Options. So Now What ? I WANT YAHOO IN THAT OPTION. What u have suggested was absolutelu useless.
Posted by: Keith Gonser at May 3, 2009 2:10 PMI have to reset Outlook Express as my email every time I turn on the computer. Can I change it permanently?
Posted by: Peg at May 11, 2009 12:40 PMDidn`t work for me. Did the messenger, the yahoo toolbar, everything, no go.
Posted by: Don at May 19, 2009 7:58 PMNot one of theses solutions worked for me. Messanger 9 does not give you those options either.
Posted by: T Williams at August 24, 2009 1:20 PMi/ve followed instructions installed gmail notifier but it is still not listed in my programs email only outlook xpress also where is downloads listed so i can see if i/ve really done it?
Posted by: greg at November 19, 2009 5:14 PM