Hi jackvdbuk,
>> I'm looking for a solution so when a event is created by a PA or the manager and they select the "send replies to" option in properties, it ONLY sends to this person and not the host.
Based on my test and research, the meeting response(accept/decline) only goes to the original organizer, not the send replies to address. That is to say, I am afraid the "send replies to" option doesn't work for meeting requests.
>> in testing it seems to send to both the host and the PA when this option is selected. the PA has send on behalf of permissions to the manager.
Do you mean when the meeting is created by the manager or the PA sends the meeting request on behalf of the manager, you want all the meeting responses only go to the PA?
If this is the case, it is suggested that the manager add the PA as his delegate, and then use the delegate settings to control the response:

Hope this can be helpful.
Regards,
Yuki Sun
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