Hotmail is very hard wit suspected spam and many factors can cooperate to mark a message as spam. The first check is to ask the tech support if the server which sends the emails (not necessarily your web server) is blacklisted and if they can setup an SPT record on the DNS to authorize the mail server to send email for your address.
Alternatively, you can use some external SMTP, like SendGrid. Or, if you provider gives you even the email, use the SMTP you use on thunderbird.
The tech support should be able to give you the best direction… copy and paste them this message.