Inboxing e-mails

Hi guys, i built an MVP of a desktop app. In order to use it users download it and create an account in the app. My backend then sends them an activation e-mail. Most of my potential customers work for one large organisation using the same Microsoft Exchange email service. After a few validation e-mails that where received without issue my e-mails don't see to get inboxed anymore. They also don't show up in the clients spam folder. Anyone have any experience resolving this kind of issue? I added a dmarc record and on mailtester.com I am now receiving a perfect score. I am using a cheap .xyz domain to send the e-mails from. Could this be the issue?