+1 with Gizmo.
Probably has to do with using cloudflare where something is amiss in the dns record.
Or possibly ssl
That is what I found for myself
I have the same issue at times.
With outlook it is hit or miss , sometimes it comes as spam sometimes not.
But I improved results using the tool I suggested.
It will also tag if the email is blacklisted from common IPv4 blacklists
