for some reason recently all emails we send that go to an outlook email will have
This sender failed our fraud detection checks and may not be who they appear to be. Learn about spoofing
What am I able to do about this?
Welcome in the world of preventing spam. Your email is rejected by a Microsoft service, so Microsoft sets the rules. According to their documentation, the cause may be spoofing or, more likely in your case an incorrectly configured Sender Policy Framework (SPF) record.
You can reproduce this problem by sending a single email to an outlook address by using just the send mail action.
My assumption is that you sent your email directly from the server where CiviCRM is installed by using the mail() or the sendMail option in Settings - Outbound Mail . But your is not registered according to the SPF framework to be a legitimate email sender for your domain.
You can try to send the emails using SMTP server of your ISP (configure smtp in Outbound Mail). Another option is to add an SPF record to your DNS. More information at OpenSPF.
However, SPF is not the only countermeasure to discourage spam. The other is DKIM.
If you are sending large volumes of email you can consider using a specialised SMTP provider.
Try sending you mailings to the email address on https://www.mail-tester.com and checking the results (just folw the big instructions on the page) - ain't no good if you get a rainy day - you need bright sunshine and birds singin'.
MSOutlook servers also often require the DMARC record set (it' tell you how).
SPF and DKIM are also good, but check the report.
SPF is easy to set up (just find some docs) - it's just registergint he address of your outgoing server - the report will also give you some details for that.