Outlook user gets a 'cannot send as' error when updating a meeting request

I have just been trying to troubleshoot a user's issue, as follows:

When trying to update a meeting request they originated, they received the following NDR message from all recipients, including an external recipient:

"You can't send a message on behalf of this user unless you have permission to do so. Please make sure
you're sending on behalf of the correct sender, or request the necessary permission. If the problem continues, please contact your helpdesk."

Within Diagnostic Information for Administrators:

#MSEXCH:MSExchangeIS:/DC=com/DC=outlook/DC=prod/DC=eurprd06:AMSPRD0610MB362[578:0x000004DC:0x0000001D]
#SMTP#

The issue appears to be similar to the following unresolved forum post:

http://community.office365.com/en-us/searchresults.aspx?q=outlook+meeting+request+NDR+external+cannot+send+as&groups=4