Outlook "Failed Fraud Detection" notice--only for some addresses from same domain
Email delivery can be a tricky business and there can be many reasons why this is happening.

To diagnose, I use https://www.mail-tester.com/

It will give you a score out of 10 and let you know where you can improve, giving you details of SPF and DKIM records which may be causing issues with delivery. 

You can just put the testing email provided by Mail Tester in the same place you put any email to send a test message in CiviMail. I'd recommend changing the from address on different tests and see how the score changes. Based on what you described, I would expect some feedback about your mail records.