- Click "Tools", then "Accounts..."
- Choose the account in problem
- Click "Properties"
- Click "Servers" tab
- Tick the "My server requires authentication" checkbox.
- Try to send an again.
If the steps above does not solve your issue, please ensure that your ISP does not block port 25. For testing, type the following command in command prompt (for Windows click “Start” > “Run” > Type “cmd” then click “OK”).
telnet yourdomain.com 25
If it gives a connection refused error message, please try to configure your email client and set the SMTP port to 26 (instead of 25). The connectivity issue usually happens when your ISP blocks this port at their servers.
Instructions of changing SMTP port in Outlook Express:
- Click "Tools" -> "Accounts..."
- Select your account, click "Properties"
- Choose the "Advanced" tab.
- Change the SMTP port to 26.
- Click "Apply" -> "OK" -> "Close"
For other email clients, please refer to their web sites for detailed instructions.