When told about it, I found this issue hard to comprehend so I went to the users computer and sent a test calendar invite out to All Staff and the Executive Group called "Please Ignore - IT Testing (please delete). Needed to see it happen for myself!
The calendar invite went out first at 9:50am, then 13 minutes later resent at 10:53am without the user clicking a button.
After spending some time examining what was going on I checked message headers. Whenever an email is relayed internally within an Exchange environment sent from OWA or Outlook, it will never have a message header. The first email at 9:50am as expected had no header. This screenshot is shown below.
Examining the second message which was sent at 10:53am does have a message header!
In the header notice under Content-Type it has "Apple-Mail".
Subject: Please Ignore - IT Testing (please delete)
Date: Wed, 2 Apr 2014 10:53:03 +0800
To: "email@example.com" firstname.lastname@example.org,
Michael Smith email@example.com,
Executive Group ExecutiveGroup@company.com,
All Staff AllStaff@company.com
MIME-Version: 1.0 (1.0)
After some more digging I found out the executive assistant has an Apple iPhone connected with Active Sync to her Exchange mailbox. Her iPhone 4 was running Apple IOS 5.1.1 (9B206) where the latest version at the time was 7.1 (11D167), her phone was well out of date.
We upgraded her phone to the latest version 7.1 and it resolved the issue. Very weird problem!