How do I create a DL matched to privileged user accounts

Question

We have created privileged Office 365 user accounts using the recommended format of username@tenant .onmicrosoft.com. The accounts are unlicensed and do not have Exchange mailboxes setup. Microsoft likes to send alerts and notifications to an email address matching the account names, so we'd like to setup a DL where the SMTP address matched a privileged account and the member is the everyday (non-privileged) account of the privileged user. We're keen to create (yet another) mailbox that need to be checked for each privileged user.

When I try assign to the SMTP address to the new DL I receive an error "This email address is already in use by an active ‎user‎ (‎Privileged Username]‎). Try a different email." I don't really understand why this error is occurring as there is no Exchange mailbox that matches the privileged username.

I'm keen to hear ideas on how we could work around this, or how others manage email send by Microsoft to privileged user SMTP addresses. Many thanks!