Messages from internal user going to Junk for only one other internal user

[Stephanie_Grammar]

Hi all,

I am just about out of ideas of what could be causing this problem and have tried everything I could think of and find when researching similar issues, so I'm turning to the Microsoft community for help.  Any other ideas here would be incredibly helpful.

The situation:

Our CMO reported to me some time ago that our COO's emails had all the sudden started getting filtered into the Junk folder.  I had them check their email rules and verify that they didn't have anything set up to move it into Junk and assisted them with going into the Junk folder, adding the COO to the Safe Senders list, and moving all the COO's messages from Junk into the inbox.  Cross fingers and hope it sticks.

Fast forward a bit, still happening.  I use the EXO Powershell module to do some further digging - no inbox rules set up, COO is still on the Safe Sender list, COO is NOT on the Blocked Sender list.  I set up an inbox rule for the CMO's account to move all messages sent by the COO to inbox. 

Now we are at today and it's still occurring.  I have removed and readded the COO's Outlook profile on his PC, every message trace I run says the mail from COO to CMO is being journaled and delivered successfully with no exceptions or warnings or any other errors.  I had him log into Outlook web to see if we could override anything from the desktop on there by way of Inbox rules or Junk filtering, but that was of no use.

The other highly strange thing is after I added the rule to move any messages from the COO into inbox, whenever the CMO would go to the Junk folder to check and move messages into the inbox (because the rule was not working), it would add them to the inbox for about 10-15 seconds, then automatically put them back into the Junk folder.  This is still happening and happens in both Outlook Desktop and Outlook Web.

This is not happening to any other users internally - not with the COO's mail being diverted to Junk or any other user with an address in our tenant's email getting filtered to another internal employee's Junk. 

I like to think I'm pretty technically literate - still just first year in IT Admin and a lot to learn, but this is boggling my mind.  I'm the sole IT resource at a small company so I don't have anyone to bounce ideas off here.  Any ideas as to what to check/look at next or has anyone else experienced this issue?

[#2]

Problem only happens for mail from COO to CMO, and for no other combination of sender and recipient in your org? The problem is at the CMO end, acting on some property of the COO mail.
The remedial Inbox rule saves the message, which then gets put back in Junk? Something else is acting on the CMO mailbox. Does the CMO have any delegates? Is there a second device involved?

[#3 · Accepted solution]

@Stephanie_GrammarHad a similiar problem a few years back at a customer.

Turned out to be a separate junk-rule in the CEO smartphone Samsung mail app. The rule was only visible and active in this app and showed neither in Outlook nor Exchange.

At one time before he clicked his assistences mail in his Samsung Mail App as "mark as spam". Everytime his smartphone did an ActiveSync-run he moved her mails in junk.

Greetings

[#4]

This was the issue! CMO has a Samsung mobile device - I checked the Spam List in the default email handler and sure enough, COO's email was the only one on the list. We removed it, moved one of the COO's emails that was lingering in Junk into the inbox and voila, it stuck. COO sent a test email and it landed in the inbox as well.

Thank you so much for the assist!

[#5]

There did end up being a second device that was causing the problem - CMO had a Samsung mobile device that he'd accidentally marked the COO as spam on, which added him to the Spam Sender list. Seems like because of the way the mailbox sync operates, the reason why it would look like it would stick and then disappear was anytime we moved it from Junk to the inbox, it would sync that update across devices and the mailbox rule from the mobile device apparently takes precedence over all others (doesn't make much sense to me as to *why* that is the case, but I'm still a newb).

Once we got that removed, everything worked as it should again.