Welcome to our forum we will do our best to help. I wonder why you are not using the Sky SMTP server to send email Outlook - Sky mail setup as it is authenticated hopefully that will work regardless of the ISP. To use our SMTP servers to send email from it it will be necessary to setup a Virgin media email address once you register, use that as the login to our SMTP server to then send from the return Sky address.
You are right in not wanting to use a Virgin Media email address. Do not even consider changing over to one! As you will see in many posts on this forum, it is something to be avoided like the plague. Insecure and unreliable!