This error occurs when your ISP provider is blocking port 25, and you are using Outlook as your email client. To correct this problem:
- Open Outlook
- From Tools, select Email Accounts...
- Select View or change existing email account and click Next.
- Select the account that you have configured to read your POP3 mail, and click Change... The Internet Email Settings dialog is displayed.
- Click More Settings, then the Advanced tab.
- Change the Outgoing Server port from 25 to 587.
- Save all settings