I am currently migrating a customer from Exchange On-premise 2010 to Office365 using BitTitan MigrationWiz and a cutover approach. One of the challenges that we see is that with this approach we have to re-create all the accounts in Office365 first before we can start using them. At this particular organization, as many other, the are several delegation and permission being utilized that we will need to re-create in Office365 Exchange online. To simplify the process I exported all the permission to a CSV and processed the data in Excel to convert the lists of users and permissions from a DisplayName to an SMTP or UPN format. With that I was able to recreate the permission in Office365 – Exchange Online.
Get-Mailbox | Get-MailboxPermission | where {$_.user.tostring() -ne "NT AUTHORITYSELF" -and $_.IsInherited -eq $false} | Select Identity,User,@{Name='Access Rights';Expression={[string]::join(', ', $_.AccessRights)}} | Export-Csv -NoTypeInformation "C:\FolderPath\MailboxFolderPermission.csv"