setup the account like normal in your cPanel.
Have their DNS server admin point the MX record to your server once all the mailboxes and forwards are set.
Now only mail is going to your server.
They should then reconfigure their "old" mail server not to keep mail for their domain locally. With cPanel Exim configuration, you take the domain out of /etc/localdomains, or with Sendmail, you take the domain out of the local-host-names file.
If the users are using "mail.theirdomain" as their send/receive mail servers in their mail programs, you will also need their DNS administrator point that name "mail.theirdomain" to your cPanel server.
If their old mail server did not use "[email protected]
" for POP3 login or relaying authentication, they will all need to change their settings so they can check the cPanel mail server which uses their E-mail address as their username.