What is the setting in your server /etc/hostname file? Based on wp_mail source code:
if ( !isset( $from_email ) ) {
// Get the site domain and get rid of www.
$sitename = strtolower( $_SERVER['SERVER_NAME'] );
if ( substr( $sitename, 0, 4 ) == 'www.' ) {
$sitename = substr( $sitename, 4 );
}
$from_email="wordpress@" . $sitename;
}
What this means is that the name localhost is pick up from /etc/hostname.