Emails sent to Hotmail are vanishing

I use two methods to work out email delivery problems.

First is to check the 'spammyness' of my emails with https://www.mail-tester.com
It gives your message/server a score out of 10, but breaks down the reason for that. I was getting below 3 on my message/server combination, but am now usually 9 or above.

Second, I use an account with SendGrid (https://sendgrid.com) to send my mail rather than rely on my own server. They have a free trial. The cool thing for me is that SendGrid tells you why your emails were bounced/blocked. Lots of other handy stuff too.