Hi ,
We have faced the same issue couple of months back to resolve this you only have limited options.
First try to create a separate segment for outlook user and send twice a week a campaign.
Secondly try to lot of A/B testing mainly on Subject line(Don’t use discount,Sale,off and other spam words) and CTA position
Lastly and most important list cleaning. Regularly clean be it on basis of bounced email and open/click-Don’t keep the profile who is not engaging with your mail.
Note-Make sure your Domain have DMARC,SPF,DKIM and try to keep spam rate low.
Let me know if this work for you😊!