Sometimes the .Mac Support site shows a System Status announcement that says "Some members may be experiencing intermittent delivery errors when trying to send mail from their mail clients. .Mac mail on the web and all other .Mac services are unaffected. Read more for interim solutions." This article explains that message in more detail.
You may experience intermittent difficulties sending mail from your .mac address using your computer's mail application.
You will be able to send mail intermittently. If you experience difficulties sending .Mac mail using a mail application on your computer, try sending your mail from .Mac Mail on the web. To access .Mac Mail on the web, go to http://www.mac.com/ and click the Mail link at the top left of the menu bar.
If you need to interact with a .Mac Support specialist, go to the .Mac Support Page at http://www.apple.com/support/dotmac/. Select the appropriate topic, fill out the form at the bottom of the page, and click "Send Question." Our support experts will make every effort to reply within 24 hours.
When the issue is resolved, you will be able to send and retrieve .Mac mail from your computer's mail application as usual.