I am registering domains on behalf of customers and need to setup email forwarding so I can setup an email address as simple as info@theirdomain.com to forward to their existing mail service (hotmail, gmail etc). We are hosting their websites at their domains but do not wish to host an email server or make them switch email providers at this time.
I have complete control over the DNS setup of their domains and a CentOS server to run some sort of mail-forwarding software on.
How do I go about setting this up / what sort of software do I need?