Hello,

Here is our workflow.  

We have a mailbox,  users email that mailbox (that mailbox is the one in use for incoming and outgoing emails).  When an email hits that mailbox, the message is redirected to our IT support distribution list.  By redirecting instead of forwarding, we see the original senders email address.   Our IT staff then replies to that email that was redirected and included our mailbox used for SDP.  None of the replies are being appended.   

When we see the headers of the sent email, it clearly has  "in-reply-to" along with the originally received message header.   This worked fine prior to moving to 365.   Any help here would be great