The first step is to configure Exchange Online to accept emails for the adatum.com domain. To do this, we add the domain in Microsoft 365. When you add your domain to Microsoft 365, it's called an accepted domain.
The next step is to tell Exchange Online what to do with those emails. You need to configure the adatum.com domain as either an authoritative domain or an internal relay domain.
Authoritative domain means that the mailboxes for that domain are hosted in Office 365. In this question, the mailboxes for the adatum.com domain are hosted on the third-party email system. Therefore, we need to configure the adatum.com domain as an internal relay domain. For an internal relay domain, Exchange Online will receive the email for the adatum.com domain and then 'relay' (forward) the email on to the third-party email server.
Reference:
https://docs.microsoft.com/en-us/exchange/mail-flow-best-practices/manage-accepted-domains/manage-accepted-domains