A split domain is when 1 domain is hosted on multiple servers (for example a POP3/IMAP server and an Exchange server).
We do not recommend this configuration, but it will be functional.
1. Configure the MX records for the domain to point to the POP3/IMAP server only.
2. Configure the AutoDiscover record as described in our KB (here).
3. Configure a secondary email address on each Exchange mailbox using the built-in domain, as described in our KB (here).
4. Configure the appropriate mailboxes on the POP3/IMAP server to forward messages to the secondary email addresses configured in step 3.
5. Remove the authoritative feature for your domain in Exchange as described in our KB (here).