I find the same issue, on several occasions I've done some wireshark / tcpdumps on Outlook (desktop) traffic and it was quite shall we say 'messy', additionally as we notice emails sent via Office365 at times go missing or just never arrive (with no NDR), a couple of us started inspecting headers of Office365 sent emails and found them to be bouncing around internal Microsoft servers with broken certificates and NTP so out of sync that exchange online (or whatever component does this for their system) moved on to another server until it got to one that was working.