Scenario: When a user sends a legitimate message to an Exchange Online recipient via Outlook Web, the email message gets moved into the Junk Email Folder. Viewing the Spam Confidence Level (or SCL) of the message header, it ranks the message with a 5, or a higher value that Microsoft will mark it as Spam. This happens consistently.
Troubleshooting: After viewing the Source code of the message of the email that went into Junk, we found it was filled with tons of blacklisted URLs. I compared this to a known good message, and the source code was much cleaner. We disabled all PluginsAdd-ins in the browser, and all emails would now consistently be delivered to the inbox of a Exchange Online user.
Cause: The plugin AdBlock Plus embeds HTML meta data into the email message when sending via OWA that identifies multiple URL’s hidden within the message body.
Solution: Disable the AdBlock Plus plugin and it will no longer embed the body of the email message with items that are picked up by Exchange Online’s message scanners.