Cannot modify header information

[SoulAssassin]

On checkout, when I do choose electronic transfer, or basicaly any function where a e-mail has to be sent,

I get this error:

 

Warning: mail() [C:\apache2triad\php\manual\function.mail.html]: SMTP server response: 550 Relay denied in C:\apache2triad\htdocs\oscommerce\catalog\includes\classes\email.php on line 522 Warning: Cannot modify header information - headers already sent by (output started at C:\apache2triad\htdocs\oscommerce\catalog\includes\classes\email.php:522) in C:\apache2triad\htdocs\oscommerce\catalog\includes\functions\general.php on line 33

 

These are the 2 line in my code:

email.php on line 522

return mail($to, $subject, $this->output, 'From: '.$from.$this->lf.implode($this->lf, $this->headers).$this->lf.implode($this->lf, $xtra_headers));

 

general.php on line 33

header('Location: ' . $url);

 

Does anyone know how else I can set this header up?

I am confused.

Thanks