Outlook 2016 mail item conversation indexes are different in sent and received emails

Question

I would expect that conversation index of the forwarded email sitting in sender's Sent Items folder is exactly the same as the conversation index of the received email in recipient's Inbox folder. In most my test cases, they are the same. However, in some cases they are different and I don't understand why. Is it something wrong in my Outlook add-in code that pre-processes the mail before sending it or it is expected Outlook 2016 behaviour? Both sender and recipient's mail boxes are on Exchange 2013 server. Please, help if you had a similar problem as well. Thanks.

Answer 1

I've decided to answer this question in case someone hits the same error. In my test, both sender and recipient mailboxes were on Exchange 2013 server. The emails were sent from Outlook 2007 and received in Outlook 2016 or the other way around. The conversation index changed its value in all tests.