A Split Domain is a domain shared between two mail servers - some users at your domain on one mail server and other users at your domain on a different mail server.
There are several situations when Split Domains could be useful. For example, during migration to Hosted Exchange from external Mail Server, when a part of mailboxes is already hosted by POA, while the other part is still hosted by external Mail Server.
It is considered that the domain is hosted outside of POA and you need to make some of POA Hosted Exchange mailboxes to be able to send and receive emails for this domain. MX records point to external mail server.
You can use Custom Primary Email Address feature to configure this kind of Split Domain hosting.
1. Create Exchange mailbox based on domain hosted in POA. It is possible to use Instant Access URL for this purpose.
2. Setup Custom Email Address for the Mailbox via Hosting > Configuration & Administration > Exchange > Mailboxes > select the mailbox here > E-mail addresses tab > press on Change in Primary E-mail Address section > select Custom e-mail address option > enter external email address to Custom e-mail address field > press on Finish button.
3. Confirm Custom Primary E-mail Address by pressing on correspondent link in confirmation email sent to external email address.
4. Setup incoming mail flow to forward to Exchange mailbox.