I am registering domains on behalf of customers and host their websites on my server.
I have complete control over the DNS setup of my customer domains. I still want to host their websites, but I don't want to be responsible for their emails
Now they need forwarding of some emails of their domains to existing mail accounts on other hosters (hotmail, gmail, etc.).
What I need is the following:
I would set the MX-record to a provider, where I can configure forwarding rules for the emails of my customers.
Would a service provider charge you for that kind of forwarding-service? There would be no storage needed at that provider, only some throughgoing traffic from the forwarded emails.
Most reputable e-mail hosting providers would allow all e-mails to be forwarded from an account to another account as well. Since most reputable email hosts charge for service its safe to assume that this type of functionality is included in the account setup.
Essentially you would setup an "account" on the new host and then forward that traffic to whatever back-end address you want it to go to. Each host has different control panels for setting this up however.
At the most basic level you could probably setup Google Apps or Office 365 for your clients to do e-mail hosting and other productivity enhancements which could provide a huge value-add for your customers. Office 365 can even be resold by you and billed monthly so it appears as part of the overall service you are providing to them.
It would take:
Yes, a service provider would typically charge for this kind of thing. Rollernet.us does this kind of email relay service, for example.