Resource mailboxes are for the scheduling of resources such as projectors. They do not require a license.
Reference:
https://docs.microsoft.com/en-us/microsoft-365/admin/manage/room-and-equipment-mailboxes?view=o365-worldwide

Reference:
https://docs.microsoft.com/en-us/exchange/security-and-compliance/mail-flow-rules/common-attachmentblocking-scenarios

Reference:
https://docs.microsoft.com/en-us/exchange/security-and-compliance/in-place-and-litigation-holds#placingpublic-folders-on-hold
Topic 1, Litware, inc
Physical Locations
Litware has a main office in Paris and branch offices in Munich, London, Brussels, Vienna, and Rome.
Each branch office has its own compliance officer.
Existing Environment
The network contains one Active Directory forest named litware.com. The forest contains a single Active Directory domain. Each office is configured as an Active Directory site.
Each branch office is represented organizational unit (OU) in an OU named Offices in Active Directory.
Each branch office has a special Administrators group that contains the delegated administrators for that office. Each Administrators group is named based on its location in a format of RegionalAdmins_Brussels, RegionalAdmins_Munich, RegionalAdmins_London, RegionalAdmins_Vienna and RegionalAdmins_Rome.
Network Infrastructure
Each office contains two domain controllers from the litware.com domain. All the domain controllers are configured as DNS servers. All client computers connect only to the DNS servers in their respective office.
All offices have a high-speed connection to the Internet. Each office connects to the Internet by using a VPN appliance.
Each office has the following servers and client computers:
The office in Brussels contains an Exchange Server 2016 server that has the Unified Messaging role installed and hosted voicemail configured.
Mailboxes are hosted on all the Exchange servers. Public folders are hosted only on an Exchange server in the main office. Litware uses a disjoint namespace to access the servers.
Each user has an archive mailbox. Archiving is configured to use the default settings.
Exchange services are published to the internet by using a reverse proxy in each office.
A full backup of Exchange is performed nightly. Incremental backups occur every six hours.
All the users in the transport department connect to network resources by using a connection in the main office.
Requirements
Planned Changes
Litware plans to implement the following changes:
Implement calendar sharing to partner companies.
Implement a solution that prevents malware from being delivered to end users.
Use personal archives to eliminate the need for PST files.
Ensure the Brussels office and move all the users in the Brussels office to the main office. All the mailboxes in the Brussels office will be moved to a server in the main office.
Implement a compliance solution to ensure that items deleted from public folders are retained. If an item in a public folder is modified, the original version of the item must be available.
Technical Requirements
Litware identifies the following technical requirements:
Minimize administrative effort.
Use the principle of least privilege.
Ensure that junk email is deleted automatically after 14 days.
Ensure that sent items and deleted items are deleted automatically after 30 days.
Ensure that users in the customer service department can open multiple mailboxes.
Ensure that all the former Brussels office users always maintain access to hosted voicemail.
Ensure that the staff in the IT department can manage the email environment without and-user involvement.
Ensure that users cannot recover email messages that are deleted from the Junk Email folder and the Deleted Items.
Ensure that if an email message has a spam confidence level (SCL) of 3 or higher, the message automatically moves to the Junk Email folder.
Problem statements
The transport department users report that accessing a public folder named TransportPF is sometimes very slow.
Users in the customer service department report that Microsoft Outlook sometimes hangs when many mailboxes are open.

Reference:
https://docs.microsoft.com/en-us/exchange/mail-flow-best-practices/remote-domains/remote-domains

Reference:
https://docs.microsoft.com/en-us/microsoft-365/enterprise/external-domain-name-system-records?view=o365-worldwide