Mailprotector provides split-domain delivery as a feature, allowing a single domain to use both CloudMail and Microsoft 365.
The option allows users who don't need all the features of Exchange Online to use a CloudMail mailbox to satisfy the basic needs for business email communication.
CloudMail is the POP3/IMAP service Mailprotector offers to give businesses a simple option for a webmail platform that's easy to configure.
To configure CloudMail with email applications, please review the CloudMail Email Client Configuration article.
Microsoft Office 365, CloudMail
Internal Emails Aren't Routing Outside Office 365
The default settings for O365 keep emails within the same domain inside Exchange Online. No attempt to send the email out to Mailprotector is made, thus failing to send.
To change this behavior, set the domain in O365 to Internal Relay instead of the default Authoritative status. See the Microsoft KB below:
For split-domain delivery to work, the Exchange server must be set to act as an Internal Relay to force an MX record lookup when a mailbox is not found.
Please contact the Partner Success team if you have more questions.