Oscommerce has no option to send email by setting any smtp account in admin interface. If you select the mail option as smtp (yourdomain.com/catalog/admin/configuration.php?gID=12&cID=125&action=edit), it will use the smtp set in the php.ini of your server. The contribution is using PHPMailer class (http://sourceforge.net/projects/phpmailer/), where admin can use any smtp account to send email. Or default sendmail also can be used by it.