Use Microsoft 365 for emails of multiple domains

You need only one active user (i.e., one user license) to receive and send emails for multiple mailboxes of multiple domains.

1. Add a domain (Microsoft 365 admin center > Settings > Domains). It is straightforward. If your domain is registered with any major registrar (e.g., GoDaddy, IONOS), adding a domain mainly requires clicking a few buttons. If your registrar is not on their list, you will need to add DNS records manually by copying and pasting the values provided Microsoft's instructions.
2. The newly added domain needs to be accepted by the Microsoft Exchange. You can check it: Exchange admin center > Mail flow > Accepted domains. The acceptance is automatic and often instant. Microsoft server sometimes has a glitch and the acceptance can be delayed, or even require their technical support's intervention.
3. Keep in mind that there is only one active users. For other mailboxes, you add them as shared mailboxes, then delegate all the tasks to the only active users. In other words, give all the necessary permissions to the active user for sending and receiving as not on behalf of the shared mailbox. This is done in Exchange admin center > Mailboxes. You add a shared mailbox, then go to the Delegation tab of the mailbox to give "Send as" and "Read and manage (Full Access)" permissions to the active user. It may take up to an hour for the setup to take effect. 
4. In Outlook, right-click the main mailbox > Add shared folder or mailbox > enter the email address of this new account. You may get an error message saying "You don't have permission to add this account. Contact the owner of the account". You usually just need to wait for half an hour