hope this helpsThank you for contacting Apple.
The first thing we would recommend is to test the account at .Mac Mail Online, <http://webmail.mac.com/>. If that works, then the .Mac services are working correctly.
Sending email using smtp.mac.com fails if your Internet Service Provider (ISP) doesn't allow proper access.
Contact your ISP:
1. Contact your Internet Service Provider (ISP) and make sure the standard port used by most email applications, Port 25, can be used to send email.
It is important to realize that many Internet Service Providers block access to the standard port (Port 25). They are not required to notify their members if they decide to do so. This discourages the sending of unsolicited commercial email or "SPAM". This can make it difficult to send email using an SMTP server other than the one used by your ISP. If an ISP does not allow access to port 25, follow step 2.
2. If your ISP is blocking port 25, you can change the port to 587 and try again. Choose Mail > Preferences, and click Accounts. Select your account, then click the Server Settings button in the Account Information pane. Change the Port from 25 to 587. Click OK, close the Preferences window, and try sending your message again.
3. If you still cannot send mail, use the ISP mail server.
Add the ISP SMTP hostname to the Mac.com account in your email application. This information can be obtained by referring to your ISP mail settings, or contacting the ISP.
The .Mac Support Team