The server is asking for your username and password

I have an issue with mi Windows Live Mail, when I select mails from one specific email address, a window appears asking me for an user name and password. It exactly says "The server wmail.kynungshin.co.kr is asking for your username and password. The server reports that is from kynungshin.co.kr. Warning: Your username and password will be sent using basic authenticationon a connection that isn´t secure."

This started yesterday, but I have mails from previous dates with the same issue now, when it did not happen before.

I read some other questions but I did not find an specific answer. I hope you can help me.

Thank you all.