Ayuda formulario, error "headers"

[tlacuilosound]

Que tal compañeros tengo un problema muy peculiar con un formulario de contacto que envía una notificación al momento de guardar los datos, me aparece estos errores:

Warning: mail() [function.mail]: SMTP server response: 503 This mail server requires authentication when attempting to send to a non-local e-mail address. Please check your mail client settings or contact your administrator to verify that the domain or address is defined for this server. in C:\Inetpub\vhosts\travelluxuryresorts.com\httpdocs \travel\sender.php on line 22

Warning: mail() [function.mail]: SMTP server response: 503 This mail server requires authentication when attempting to send to a non-local e-mail address. Please check your mail client settings or contact your administrator to verify that the domain or address is defined for this server. in C:\Inetpub\vhosts\travelluxuryresorts.com\httpdocs \travel\sender.php on line 24

Warning: Cannot modify header information - headers already sent by (output started at C:\Inetpub\vhosts\travelluxuryresorts.com\httpdocs \travel\sender.php:22) in C:\Inetpub\vhosts\travelluxuryresorts.com\httpdocs \travel\sender.php on line 25

La verdad no tengo ni idea cual se el problema, nota, cuando escribo una direccion perteneciente al dominio funciona bien, pero un hotmail o gmail..... prrr...