Grant Exchange Calendar Rights

To view Calendar access rights:
Get-MailboxFolderPermission -identity "ManagingUser:\Calendar"

You must remove any user listed as Reviewer before you can add Editor rights.

Remove-MailboxFolderPermission -identity “ManagingUser:\Calendar” -user “PersonalAssitant”

Once Removed Add Editor Access rights:

Add-MailboxFolderPermission -identity “ManagingUser:\Calendar” -user “PersonalAssitant” -AccessRights Editor

Grant Mailbox Permission to single user

 There are many situations in which permissions to another user’s mailbox should be granted on Exchange Server. Sometimes it is for monitoring purposes, sometimes in order to send emails on behalf of someone else. Full access permissions give the highest level of access to a mailbox, and are necessary, for example, during a migration process. If you want to learn how to set full access permissions on Exchange 2007, Exchange 2010, Exchange 2013, Exchange 2016 or Exchange 2019



Get-Mailbox -ResultSize unlimited -Filter {(RecipientTypeDetails -eq 'UserMailbox') -and (Alias -ne 'User')} | Add-MailboxPermission -User User@Example.com -AccessRights FullAccess -InheritanceType all -AutoMapping:$false

Full Credit to:

https://www.codetwo.com/admins-blog/how-to-grant-full-access-permissions-to-all-users-mailboxes-exchange/

PowerShell-Bulk Export & Import of X500

One of the most important thing to take care is the X.500 Address when you are doing Direct Cut Over migration from Microsoft Exchange Server to a new Exchange Server in different AD Domain or Office 365

The X.500 Address is used by Microsoft Outlook to identify users (NOT SMTP Address) within Exchange Organization (Refer to the http://www.askme4tech.com/exchange-server-x500-address-amazing-thing-know for more detail information.

Please refer to the steps below on how to export & import X.500 using PowerShell Script
To export the ExchangeLegaceyDN from Source Domain. 

 Get-ADUser -SearchBase “DC=YOUR-DOMAIN,DC=COM” -Filter * -Properties SamAccountName,legacyExchangeDN | Select-Object SamAccountName,legacyExchangeDN | Export-CSV C:\Temp\Exported-LegacyDN.csv -NoTypeInformation

To Import to new Domain Controller. 

Import-Module ActiveDirectory
$Input = Import-CSV C:\Temp\Exported-LegacyDN.csv
ForEach ($ADUser in $Input){
if ($ADUser.legacyExchangeDN){
Set-ADUser -Identity $ADUser.SamAccountName -add @{proxyAddresses=”X500:$($ADUser.legacyExchangeDN)”}
}
}

All credits:

https://aventistech.com/2018/07/powershell-bulk-export-import-of-x500/

Source for the Script.

https://www.petenetlive.com/KB/Article/0001468

This describes my exact issue:
https://www.tipsdotcom.com/recipient-not-found-by-exchange-legacy-encapsulated-email-address-lookup.html