getaddrinfo: Name or service not known

Hi   I have just tried using ActionMailer to send mail in rails.For that i followed code from railcasts.And it worked also.But some times i am getting the error "getaddrinfo: Name or service not known"..Why this happens?

Please help Sijo

http://blog.innerewut.de/2007/9/23/name-or-service-not-known