[EWS] Setting Organizer for calendar entry

I've got an application that adds calendar entries via EWS using direct XML, not the API.  I am trying to add an event to a user calendar, where the user is an attendee, but not the organizer.  If I try to set the <t:Organizer> property, I get the error response "Set action is invalid for property".  I can set the attendees, but the organizer always comes out to be the person who's calendar I am updating - which is inaccurate.  I might understand this if the organizer was some random name/address, but in this case, I am setting the name and email to be that of an existing Office 365 user in the same domain.  Any ideas?

Here is a xml snippet showing my code:

...
<t:Organizer>
<t:Mailbox>
<t:Name>
Jamie Thorogood
</t:Name>
<t:EmailAddress>
css02jt@gold.ac.uk
</t:EmailAddress>
</t:Mailbox>
</t:Organizer>
</t:CalendarItem>
</Items>

Jeffrey McKay