When someone replies to my emails, their reply get a faulty time stamp. Often their reply has a time stamp earlier than the initial message they replies to. This leads to the threads showing the correspondence get totally faulty and the order of the messages wrong.
Did you check the server time of your e-mail server? This can be a reason for this. Maybe the settings for time offset (general mean time, european summer time, a.s.o.) is set wrong. I would investigate this.
Open one of these emails an look on the source code of it (Properties – Source Code).
Check the time entries like 25 Oct 2018 11:03:35 +0200 (CEST) and try to find out, where the time is set wrong. This may give you and idea, which time is confusing your in-box and who should fix the time (I expect sender server time).