I cannot send emails because my internet service provider is blocking port 25

In order to avoid receiving spam mail some internet service providers are blocking port 25. That is why you have the possibilty to use port 10025 instead.

You do this as follows:

Choose the drop-down menu Tools -> Accounts:

Choose the tab "Mail". Click on the account where you wish to change the servers port number and click on "Properties":

Choose the tab "Advanced".

In the field "Outgoing mail (SMTP)" you change 25 to 10025.

Click on "OK":

This description is based on Windows Mail. But the settings are similar to its precursor Outlook Express.