If you can receive but cannot send email, it’s likely your ISP is restricting access to SMTP on port 25. This article shows you how to change the default SMTP port number in Microsoft Outlook Express, Microsoft Outlook, Mozzila Thunderbird, Microsoft Entourage and Apple Mail.
Microsoft Outlook Express
1. Open Outlook Express.
2. Click Tools > Accounts.
3. Click the ‘Mail’ tab and double click on your account.
4. Select your account
5. Click the ‘Advanced’ tab and change ‘Outgoing Mail server’ to 26.
6. Click the advanced the advanced tab and change port to 26
7. Click OK and follow prompts to return to Outlook.
That’s how you can change SMTP port to 26 in Outlook.