How to investigate "spam" messages delivered to Junk Mail

1. View a message in the Junk Mail folder using Outlook and check the message info in the header area.

“This message was marked as spam using the Outlook Junk E-mail filter” indicates that Outlook identified the message as spam. Check your settings under Home > Delete > Junk > Junk E-mail options…

“This message was marked as spam using a junk filter other than the Outlook Junk E-mail filter” or no spam marking information indicates that something other than Outlook identified the message as spam. The most common causes here are:

1. The server-based junk mail filter “Exchange Online Protection” (EOP) 
2. An Outlook rule that moved the message to Junk Mail after it had been delivered

1. Cut-and-paste the message headers into the Message Header Analyzer
2. Examine the X-Forefront-Antispam-Report header:
   

   Parameter	Meaning
   SCL	Records the final decision on whether something was spam or not
   SFV	Records the decision based on message content filters
   CAT	Records the categorisation decision, which can identify other types of undesirable message
   IPV	Not useful as filtering based on external sender address is done elsewhere in our system

3. Examine the X-Microsoft-Antispam-Mailbox-Delivery header:
   

   Parameter	Meaning
   dest:J	Server-side decision to deliver to Junk Mail
   dest:I	Server-side decision to deliver to INBOX
   dest:C	Server-side decision to deliver to user-specified (custom) folder
   OFR:CustomRules	Outlook rule determined delivery of this message
   OFR:SpamFilterAuthJ	Exchange determined delivery of this message, possibly using mailbox spam rules

Notes

Ignore IP filtering results or SPF failures in the Authentication-Results and related headers. Sender-IP filtering is done elsewhere in our system, and the headers you see are not based on the actual sender.

In Outlook, check Home > Rules > Manage rules. Check message rules in both the Outlook desktop application and Outlook on the Web to ensure you have checked all rules.

Details of the MS headers are available at Anti-spam message headers - Office 365 | Microsoft Docs

When checking any message delivery it is important to note the different sender addresses used. These are recorded in message headers, are all optional and include:

Differences in the sender address domains can adversely affect message reputation. Links in the message body to other domains (not in the sender addresses) can have the same effect. This makes it more likely that a message will be classified as spam/junk.

X-NTG-DKIM-verify and X-TM-Authentication-Results indicate the outcome of SPF and DKIM verification on the message by TrendMicro EMS. A failure here indicates that the sender address could not be validated or failed validation, providing an indication that the message was not really sent by the stated sender or that the sender’s mail configuration is incomplete or incorrect.