This article explains how to migrate shared mailboxes between Office 365 tenants.

What is a shared mailbox?

Shared mailboxes can be accessed by more than one user. They allow members of the same team to share a mailbox and share the responsibility for reading and responding to messages.

Shared mailboxes are standard mailboxes with email addresses assigned to them.
However, you don’t need to assign licenses to SharedMailboxes, because the users of the organization who monitor and use the SharedMailbox use their own license to access the shared mailbox.

Permissions on shared mailboxes

One challenge in migrating a shared mailbox between two systems is to maintain the permissions (specifically who can access the mailbox and who can send mails).
Typically, there are two kinds of permissions that need to be migrated — Full Access and Send As.

From the Help in the Exchange Management Console:
Manage Send As Permission
Click Manage Send As Permission to open the Manage Send As Permission wizard. This wizard will help you grant Send As rights for this mailbox to other users or groups, or revoke Send As rights from users or groups that were previously granted this permission. When you grant Send As rights for a mailbox to another user, that user will be able to send messages as that mailbox.

Manage Full Access Permission
Click Manage Full Access Permission to open the Manage Full Access Permission wizard. This wizard will help you grant full access rights for this mailbox to users or groups, or revoke full access rights from users or groups that were previously granted this permission. When you grant full access rights for a mailbox to another user, that user will be able to log on to that mailbox and access its entire contents.

Migration of share mailbox permissions

Cloudiway provides a set of scripts to recreate shared mailboxes in the target and maintain its permissions. Please contact us at support@cloudiway.com to get these scripts. Please note that they are reserved only for customers who are doing the migration with our platform.
1. DumpSharedMailboxes.ps1 : this script connects to the source environment (Exchange or Office 365) and lists all available shared mailboxes along with their Full Access and Send As permissions. It dumps all the information into a CSV file.
2. WriteSharedMailboxesPerms.ps1 : it reads the CSV file generated by the first script and recreates the Shared Mailboxes in the target and sets the initial Full Access and Send As permissions.

Once the shared mailboxes are created, you are ready to migrate their content using Cloudiway Migration. All you need to do is to define a migration entry as if it were a standard user (create a user in the user list and just define the source email and target email address).

See this article for more information.