When a user modifies a migrated calendar item, the update is refused with the following error:
'Migrated User' on 7/13/2017 9:21 AM This message could not be sent. Try sending the message again later, or contact your network administrator. You do not have the permission to send the message on behalf of the specified user. Error is [0x80070005-0x0004dc-0x000524].
Add the old Exchange server's LegacyExchangeDN to the user's account as an X500 address.
Example: /O=<ExchangeOrg>/OU=FIRST ADMINISTRATIVE GROUP/CN=RECIPIENTS/CN=<Migrated User>
See the following Microsoft solution for more information: