A doamin name is held by another hosting company and the client wants the email to continue on that hosting company but are happy for you to host the web site on anther hosting company e.g. Siteground
1.Firstly, the domain that will be hosted with Siteground can be added as a domain addon.
2. Then it will appear in the Advanced MX editor in the cPanel
3.From the Advanced MX Editor you can change these records to point to PlusNet etc (their support can give you the correct MX settings)
4.And then, once the content is here on the SiteGround servers you can change the name server records for that domain directly to our name servers. Since the MX will already be present in the cPanel, the domain will connect to the PlusNet etc email services