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First try restarting your computer. If the error persists try removing the mail account from your mail client, close it, and re-add it.
Email Configuration Tutorials
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 software you have installed is blocking your access.
Now as an available option you may use port 50 as an alternative port for sending mail via SMTP.
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