Outlook - Received time changed automatically for some emails.

Hi, 

I got a few frantic phone calls this morning, from people saying that they had opened their email this morning to find that they had a list of emails at the top of their inbox with Received times of 4-5am. They said that these emails were actually duplicates of emails already received earlier this week. 

I checked the Mail Flow and there was no log of these mails having been sent / received at the Received times that were stamped on the emails. Having added a few extra fields in Outlook I noticed that the Sent time for the emails was Tuesday, but the Received time seemed to have changed to times of between 4 and 5 am this morning. 

Date changes

So it seems that the mails were not re-sent they simply had their Received time changed. This occurred for multiple users but only for a small number of emails. 

Has anybody else had this issue or can anybody offer a reason as to why this might have happened?

thanks,