This is geared toward Microsoft Outlook 2007, but if you’re on another version, you should be able to follow along fairly well anyway.
- Click Tools > Account Settings
- Double-click the account you want to edit
- In the window that pops up (also known as the “Change Email Account” window), click “More settings …”
- Click the “Advanced” tab
- Change the “Outgoing server (SMTP)” setting to 587 (or to 25 if you’re already on 587 and having problems)
That should do it. If that works it means your ISP is probably blocking port 25.