You will come across the issue with leavers mailboxes needing to be transferred to another users mailbox. If you are looking after on-site exchange, then the process is slightly shorter as you don’t need to connect to the remote Powershell.
To connect to O365 first run this PS command:
$Cred = Get-Credential
$s = New-PSSession -ConfigurationName Microsoft.Exchange -ConnectionUri https://ps.outlook.com/powershell -Credential $cred -Authentication Basic –AllowRedirection
PS command to transfer mailbox items:
Search-Mailbox -Identity “Old User” -TargetMailbox “Current User” -TargetFolder “Old-Mailbox” -LogLevel Full