Hello,

I have just been through a multi-week support call on a very odd issue. I've got it solved and it took MS Support to help me to do it. In order to share what I've learned, I am posting here.

Issue: Delegate cannot accept/decline meeting invitations using Mac Outlook 2011.

Delegate can use other applications such as Outlook (Windows), OWA, or Mac Mail to perform delegation duties. Outlook 2011 invitation responses, however, get stuck in the Outbox. Outlook shows an -18500 error.

Consider this scenario:

1. Exchange Administrator grants User B Full Access to User A's Exchange Mailbox.

2. User A sets up User B as a delegate.

3. User B uses Exchange Web Services exclusive client to perform delegate duties.

According to a tech on the Office for Mac team, this situation has the potential to cause permissions conflicts that only affect EWS clients (of which Mac Outlook 2011 uses EWS exclusively). The tech I spoke with says he's "seen it twice."

Solution: Remove User B's Full Access permissions from User A's mailbox.