I have used these e-mail addresses with win7,8 and 8.1 with no problem. It has only started in win 10. i have downloaded Mailbird to receive my e-mails and it is working fine with these addresses so...

Hi my e-mail are set out as "firstname_surname@live.co.uk" and i get this on mine and the wifes on @hotmail.co.uk

Unable to send or receive messages for the Live.co (firstmame_surname) account.
...

Although live mail does work in windows 10 it appears to depend on e-mail address as i have two that
use the underscore _ in the name and these have fail since installing windows 10.