First try restarting your computer. If the error persists try removing the mail account from your mail client, close it, and re-add it.
You will also want to make sure your Internet Service Provider is not blocking port 25 access outside their own service. By opening a shell/command prompt and typing "telnet mail.yourdomain.com 25" you should be able to reach it. If not, your ISP or firewall software you have installed is blocking your access.
