How can I set up a custom domain name using iCloud's email services?

Apologies. I read your original question very differently than intended.

From what I am aware, the answer is yes. You can use the service for business purposes. However, there are limitations you should be aware of. See here: Use Custom Email Domain with iCloud Mail - Apple Support

The key limiter is item one:

You can use up to five custom domains, with up to three personalized email addresses per domain.

If you have more than three people per domain, the Apple solution may not be for you.

Also, if you are looking to do Managed Apple IDs, you will see they are not supported. This is because you will not be able to do a federation to an identity provider.

Sorry for the confusion on the initial response.

Are you asking if you can create emails like marek@apple.com? No. Absolutely not. apple.com is the property of Apple and you must be hired by Apple to have an email address using that domain. And if you were an employee, you cannot use that email domain for a business unassociated to Apple.

If you question is about the iCloud service and the included email, I recommend that you use your business domain. There is nothing more unprofessional of a business than using a bulk email address. (For example, gmail.com, hotmail.com, iCloud.com, etc.). This often suggests that your business is not really mature and you don't have a proper digital identity.

If you are starting a business, register a domain name. Once you have a domain name, setup your digital identity by creating a web site and setting up mail. If your business is SomeAmazingBusiness.com, then your email should be marek@someamazingbusiness.com.

Take you business seriously. Get a domain and use an email address the reinforces your business identity.

Hope this is helpful.