Let me guess at what is going on:
You click on a MailTo link on a web page, and it brings up Windows
Mail, which then complains that your server name is invalid?
I see you posted with a Yahoo email address. Is that your
only email address? US-based free Yahoo accounts have webmail access
only, no POP access. Windows Mail handles POP mail but not Yahoo webmail.
Yahoo's solution is to pay them for premium "Mail Plus" service:
http://help.yahoo.com/l/us/yahoo/mail/original/mailplus/overview/index.html
and use the special settings for that service:
http://help.yahoo.com/l/us/yahoo/mail/original/mailplus/pop/pop-14.html
If you are OK with using a non-Yahoo email address, you have these other
solutions that will work with Windows Mail:
* Use the email address assigned to you by your ISP.
* Use a free Gmail, AOL, or Hotmail/Live account.
You can also continue to use Yahoo's webmail:
http://mail.yahoo.com
and just copy and paste the MailTo link from the web page.
If you need more help with any of these solutions, just ask.
Incidentally, it is best not to publish your actual email address on a forum
like this one, because spammers will collect it.