Email failure when somebody register

When somebody register in my site, automaatically it been sent a mail but it faile, how activate this funtion on my site?

Hi @fidel_c_d,

Which software and version are you currently using?

Kind Regards,
Patricia M.
Public Knowledge Project

I´m using version, and it say automatically that wee have sent an e-mail but nothing. When trying to send the mail manually it say ok but nothing happen

I think, maybe there is something wrong in the site configuration but I don´t know what