It's pretty easy:
$message =" You received a mail from ".$mail;
$message =" You received a mail from ".$name;
$message .=" Text of the message : ".$text;
Add the concatenation operator to trhe second line, like you have in the third:
$message =" You received a mail from ".$mail;
$message .=" You received a mail from ".$name;
$message .=" Text of the message : ".$text;