Outgoing mails sent to Microsoft accounts are flagged as Spam

Question

All outgoing emails sent from my own domain, cardona.pt, keep being flagged as spam and are sent to junk on microsoft emails.

I've done my due diligence and ensured that no issues were found using https://mxtoolbox.com/ that could explain why my emails are being flagged as spam, this issue doesn't happen with google or any other provider.

My emails aren't marketing or anything similar, it's just something for my own personal use so I have no idea of how to fix it as Microsoft own support form implies that I must be using my domain in some kind of business manner.