This is a common issue with Office 365 / Hotmail / Outlook and sometimes Gmail.
Do your best
Unfortunately, spam filters are largely black boxes. Therefore, there little you can do other than do your best.
Make sure to try to get a 10 out of 10 score from mail-tester.com. It’s simple:
Go to mail-tester.com, copy the email address given to you, send an email to it, wait a few seconds, then click the button to check your score.
Some factors that might impact your spam likelyhood are DNS, Domain reputation, server reputation, and the content of your email.
You can also report a false positive to Microsoft