How to Send mails in asp.net without giving authentication

I want to sent mails from my application, from address may be any domain and  to address also any domain.But in my application i already registered my domain name(mail.bsl.co.uk) in web.config file.the mails are going from that domain(xyz@bsl.co.uk) to other domains.But i want to sent mails from gmail,yahoo etc to other domains.