Customer has a website at www.theirdomain.com . However they do not, for whatever reason, use email through their site. They perhaps use Gmail or Hotmail or their ISP email addresses.
Is it possible for me to sell them email hosting so they have [email protected]?
Yes, you can create a Mail Plan in your control panel using E-Mail plan wizard and then can provide email hosting to that customer.
After adding the domain in this plan, you’ll have to setup custom MX records on the other end whose name servers are being used and then client can create the email id’s in control panel and use that.