ca.mg4.mail.yahoo.com is a mail server for the Canadian branch of yahoo. It looks like they changed the server address or are doing extended maintenance work on it. Happened on German yahoo, too, a few years ago so that I couldn't use pop3 anymore until I changed the settings to the new server address.
It's nothing to worry about, just a huge nuissance. You can access your mails using yahoo.com or any other national yahoo (like e.g. yahoo.co.uk) and your usual username and password. They all get their info from the same database so that you can read mail from all your accounts on any yahoo site.
If you used the ca.mg4.mail.yahoo.com for a pop3 mail access (that means downloading the mails directly to your computer with a mail software like for example outlook), then go to your yahoo site and have a look at their help index. The new pop3 settings for your operating system and mail software will be listed there.