So, not a huge issue, since one of the parties is out of town right now but I wanted to run it by people to see what they thought.
This has happened a total of two times and both times have involved sending the e-mail from User X to an internal recipient. However, when the recipients get the e-mail, it appears to have come from User Y.
We use Mimecast but the first time it happened was before Mimecast was implemented.
When I do a message trace in O365, I see the messages and they appear to come from User Y but they actually came from User X.
When I look at each user's mailbox in Outlook, neither user actually has the e-mail in their sent items (not that it couldn't have been moved or deleted) but I doubt they were organizing or moving this e-mail.
I can tell from the message header that this message came from User X because the first hop in...