If an email is filtered as spam inside Hotmail this is because the spam filter has perceived the email to be spam either looking at the subject line or content of the email in general. 

You can alter the spam filter settings inside your Hotmail email client by doing the following:

1. Open a message from the desired sender.

2. Click Mark as safe next to the From name and address.

3. Now click Add contact.

By doing the above you are ensuring the future emails sent form the sender will land inside your inbox rather than spam folder.