And also this.
I can't send mail
If you're having trouble sending mail from the mail application on your computer, there are a few things you can try.
If you can't send mail:
Make sure you're connected to the Internet. Open your web browser and see if you can view a page such as
www.apple.com.
If you can connect to the Internet, go to
www.mac.com, and then click Network Status (under Support) to check for any email system outages. If there are problems with the Apple web server, you will see an alert informing you of a temporary outage.
If you recently changed your member password, open your email application and change the password for your .Mac Mail account as well. Also, if you're using Mac OS X version 10.3 or later, open System Preferences, click .Mac, and type your new password; or, if you're using a version of Mac OS X earlier than version 10.3, open System Preferences, click Internet, and type your new password on the .Mac or iTools tab.
Check your email program settings and make sure you are using either your ISP's SMTP server or smtp.mac.com. To find out your ISP's SMTP server address, read the information you received when you signed up for your account, visit your ISP's website, call its support line, or look at the settings for another email account that uses your ISP's server.
If you use smtp.mac.com for your SMTP server, make sure you enable SMTP authentication in your email program using your .Mac member name and password, and make sure the email address in the From field of your email message matches your .Mac member name. If the trouble continues, switch your email program to use your ISP's SMTP server instead.