I can’t send email because my internet provider blocks port 25.

To prevent spam, some internet providers block port 25 on the outgoing mail server (SMTP).

That’s why you have the option of using port 10025 instead.

Most email programs have a field where you enter the port. In other cases, such as mobile phones, you enter the port by adding :10025 to the outgoing mail server – so if your domain name is webshopdemo.dk, the outgoing mail server (SMTP) becomes mail.webshopdemo.dk:10025

See also the guides to setting up email on the email service support page.