I've been told to look at DKIM and SPF
I assume DKIM is set, under cpanel > email >Authentication > DKIM is enabled.
My SPF record is this
v=spf1 a mx include:websitewelcome.com ~all
headers on email.
Authentication-Results: spf=softfail (sender IP is 192.1.1.1)
smtp.mailfrom=mydomain.com; outlook.com; dkim=pass (signature was
verified)