Citar:Here is a compilation of 3 posts that helped me fix this issue:
After deleting and recreating an account in Mail, you might not be able to send emails even if you can receive them. When you try to send a message, this alert may appear: "Cannot send message using the server smtp.xxxxx.com"
This happens if the Simple Mail Transport Protocol (SMTP) server, used for outgoing messages, was incorrectly entered with the first account even if it was correctly entered for the replacement account.
Resolution
You should first verify that the network connection you're using is the correct one for the ISP that you want mail to communicate with.
1. Open Mail.
2. From the Mail menu, choose Preferences
3. Click Accounts.
4. On the Outgoing Mail Server (SMTP) pop-up menu, choose Edit Server List.
5. Delete all entries for the SMTP server that is mentioned in the error
7. Send a test message to your email address.

In Mail > Preferences > Accounts > Account Information > Server Settings, set the outgoing mail server for your .mac account to whatever your ISPs outgoing mail server is. for example - smtp.mac.com used by XXXX@mac.com
Use server port 587 (make sure thats open in your firewall for TCP/UDP)
From http://www.apple.com/support/
Quote: Note: Some ISPs restrict access to the TCP port (25) used by SMTP. If your ISP is blocking port 25, you can use your email application to change the port to 587 and try again. If you continue to have trouble sending mail using smtp.mac.com, switch back to your ISP's SMTP server.