Permissions issue with pre-migration calendar entries

We recently migrated a client from an in-house Exchange Server (2010) to Office 365. We exported the mailboxes to PST using the workstations and imported the data back into their Office 365 profiles.

We now find that calendar entries made prior to the migration are causing issues. Opening up an entry and re-sending meeting appointments to the attendees results in an error as follows:

From: System Administrator
Sent: 20 May 2015 14:27
To: Joe Bloggs
Subject: Undeliverable: MEETING APPOINTMENT NAME

Your message did not reach some or all of the intended recipients.

Sent:      20/05/2015 14:27

The following recipient(s) cannot be reached:

'Fred Soap' on 20/05/2015 14:27
This message could not be sent. Try sending the message again later, or contact your network administrator. Error is [0x80070005-00000000-00000000].

Can anyone shed any light on this issue? The error appears to suggest that the original author of the calendar entry is the previous server's user account - The new user account on Office 365 doesn't have permission to re-send this entry.

New entries work fine - it's just ones that have been imported.
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Vasil Michev (MVP)Commented:
The on-prem and the cloud mailboxes are two different objects, none of the permissions you had previously apply. In addition, calendar items come with unique IDs, which will most likely also not match. Just recreate them.
edz_pgtAuthor Commented:
Thanks Vasil - perhaps your could expand on this please.
What steps do we do to recreate them?
Vasil Michev (MVP)Commented:
What I mean is to recreate the meeting by selecting the participants/notes from the already existing one.
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edz_pgtAuthor Commented:
Are you seriously suggesting that the only answer is to tell our clients to "just recreate all their calendar entries"?

There must be something somewhere to take ownership of these calendar entries. A Powershell script maybe?
Could you try to modify one of the meeting by insert the sender/recipient address from Office 365 GAL? then send metting out and verify if the issue still persist
edz_pgtAuthor Commented:
Thanks for the reply.

I've tried editing a calendar entry using OWA. I've deleted the contacts and re-entered them but I'm getting the same issue. I presume that using OWA will rule out any cached email address issues with the previous server?
edz_pgtAuthor Commented:
I've sorted it myself.

For anyone else that bumps into this thread, you'll need to add the old server user's X500 email address to the Office 365 profile. Simple(!)

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edz_pgtAuthor Commented:
It seems nobody at Experts Exchange knew this simple bit of information - frustrating since this has taken several hours to diagnose myself :-/
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