authenticating 3 IPS for a mail server(linux)

We have a VPS (email server) for sending transactional emails. it accepts outgoing transactional emails from only one IP that is the website server's IP We need to add 2-3 more ips as we are soon shifting our website server to new subnet 1) so we basically need to add the new ips 2) we need to configure SPF and DKIM correctly to be passed by hotmail. Thanks.