"This sender failed our fraud detection checks" is when the SPF record intended to validate that there is no E-mail spoofing fails. Basically the SPF record says "If you don't believe me, go ahead and Ask blah.com"; the "Fraud detection" message appears if, when Outlook reaches out to blah.com, it says "I've never heard of that user". So Outlook shows a warning, as that is a common tactic used to spoof E-mails by scammers.
It might not show the message if you add the recipients in question to the "Safe Senders" List in Settings (Gear icon in the upper right->Settings)