Reference:
https://www.enowsoftware.com/solutions-engine/what-are-your-exchange-hybrid-options

Reference:
https://docs.microsoft.com/en-us/exchange/mailbox-migration/migrating-imap-mailboxes/migrate-other-typesof-imap-mailboxes

Reference:
https://docs.microsoft.com/en-us/powershell/exchange/find-exchange-cmdlet-permissions?view=exchange-ps
Topic 1, Contoso Ltd
General Overview
Contoso, Ltd. is a national freight company in the United States. The company has 15,000 employees.
Physical Locations
Contoso has a main office in Houston and 10 branch offices that each contains 1,000 employees.
Existing Environment
Active Directory and Microsoft Exchange Server Environments
The network contains an Active Directory forest named contoso.com. The forest contains one root domain named contoso.com and 10 child domains. All domain controllers run Windows Server 2019.
The forest has Active Directory Certificate Services (AD CS) and Active Directory Federation Services (AD FS) deployed.
You have a hybrid deployment of Exchange Server 2019 and Microsoft Office 365.
There are 2,000 user mailboxes in Exchange Online.
Each office contains two domain controllers and two Mailbox servers. the main office also contains an Edge transport server.
The organization contains 100 public folders. The folders contain 80 GB of content.
All email messages sent to contoso.com are delivered to Exchange Online. All messages sent to on-premises mailboxes are routed through the Edge Transport server.
Advanced Threat Protection (ATP) is enabled and configured for the Office 365 tenant.
Network Infrastructure
Each office connects directly to the Internet by using a local connection. The offices connect to each other by using a WAN link.
Requirements
Planned Changes
Contoso plans to implement the following changes:
For all new users in the on-premises organization, provide an email address that uses the value of the Last Name attribute and the first two letters of the First Name attribute as a prefix.
Decommission the public folders and replace the folders with a Microsoft 265 solution that maintains web access to the content.
Technical Requirements
Contoso identifies the following technical requirements:
All email messages sent from a SMTP domain named adatum.com must never be identified as spam.
Any solution to replace the public folders must include the ability to collaborate with shared calendars.
Security Requirements
Contoso identifies the following security requirements:
The principle of least privilege must be applied to all users and permissions.
All email messages sent from an SMTP domain named com to contoso.com must be encrypted.
All users must be protected from accessing unsecure websites when they click on URLs embedded in email messages.
If a user attempts to send an email message to a distribution group that contains more than 15 members by using Outlook, the user must receive a warning before sending the message.
Problem statements
Recently, a user named HelpdeskUser1 erroneously created several mailboxes. Helpdesk1user1 is a member of the Recipient Management management role group.
Users who have a mailbox in office 365 report that it takes a long time for email messages containing attachments to be delivered.
Exhibit

Reference:
https://docs.microsoft.com/en-us/exchange/security-and-compliance/messaging-records-management/mailbox-retention-hold

Reference:
https://docs.microsoft.com/en-us/exchange/permissions-exo/feature-permissions
https://docs.microsoft.com/en-us/exchange/collaboration-exo/public-folders/set-up-public-folders