Microsoft / Office365 / Outlook / Live / Hotmail will sometimes add a warning message, like the following, on emails from your mailing list.
[This sender failed our fraud detection checks and may not be who they appear to be. Learn about spoofing at ...]
The message is placed at the top of the email. It does not block the email, but it may confuse the recipient.
You can disable the messages by following the steps below.
1. Navigate to the Exchange Admin Center
2. Navigate to the "mail flow" section
3. Add a new rule
4. Select "Bypass spam filtering..."
5. Name the rule "stellar.aero" (without quotes) and select from the "Apply this rule if..." dropdown, the "The sender..." -> "domain is" rule.
6. Add the domain "stellar.aero" (without quotes)
7. You should see the domain added to the list and click "OK".
8. Ensure the "Apply this rule if..." is set to the rule that was just created.
8a. Select "Header or envelope" from the "Match sender address in message:" dropdown.
9. Ensure the "Do the following..." section is set to "Set the spam confidence level (SCL) to..." -> "Bypass spam filtering"
10. Ensure the rule has been added with the appropriate settings that were just set.