NDR Error Updating Calendar. appointments/meetings (4248591)

Post migration when updating Calendar appointments/meetings, an error is generated as follows:
 

• 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. 

• The object is on-premise mastered and it is unable to set the x500 on the target

When you use ODM to create accounts on the target tenant, it automatically adds the LegacyExchangeDN / X500 address of the source user as a proxy address on the target user. This allows the target user to receive emails sent to the source user’s address.

However, ODM only migrates the LegacyExchangeDN / X500 address of the source mailbox (cloud) to the target mailbox (cloud) during the mailbox migration process. If your source or target environment has an on-premise Active Directory, you need to migrate the LegacyExchangeDN / X500 address of the on-premise objects separately. ODM Directory Sync can be used to bring over the X500 address, if you do not have licenses for Directory Sync you can reach out to Quest Sales or your Quest Account Manager to discuss..

To check if the migration was successful, follow these steps:

• Go to the Exchange Online admin portal and select a mailbox of an affected user.
• Look at the list of email addresses and see if the source LegacyExchangeDN / X500 address is there. It should look like this: /O=/OU=FIRST ADMINISTRATIVE GROUP/CN=RECIPIENTS/CN=
• If the source LegacyExchangeDN / X500 address is missing, add it manually to the target user’s account as an X500 address. Do the same for any shared mailboxes that the user has access to. This will prevent errors when updating shared mailbox calendar appointments.

For more information, see this Microsoft solution:

https://support.microsoft.com/en-us/help/3131416/-0x80070005-0x0004dc-0x000524-error-when-a-user-sends-updates-to-calen