Microsoft Exchange: difference between a "mailbox" and a "mail user"?

I'm in the process of migrating from G Suite to Office 365 using the article on Microsoft's website. It asks you to create a Mail User for everyone in the organisation, however, we already have a Mailbox for each user in the system (automatically created by Active Directory in Azure). Do you know if it's really necessary to create a Mail User when we already have a Mailbox for each user?

Assuming it's not, do you by any chance anyone know how to set the "ExternalEmailAddress" on a mailbox in Exchange? I can see such an option when creating a mail user but not when creating a mailbox. This address is used to forward email back to G Suite during the transition period. I'm assuming it's not the same as a standard email forwarder as the article specifically uses the term "ExternalEmailAddress". Thanks for your help.

Mail users are similar to mail contacts. Both have external email addresses and both contain information about people outside your Exchange Online organization that can be displayed in the shared address book and other address lists. However, unlike a mail contact, a mail user has logon credentials in your Microsoft 365 organization and can access resources.

User mailboxes are people all in your organization, there are not any external information.

There is no option to configure external email address for a usermailbox, you could configure inbox rule for this mailbox to forward received mails to external emails addresses.