The junk mail filter in Outlook is purely a content filter. If you are finding your messages landing in the junk mail folder often, it indicates there is an issue with the way you are wording the content within. Try to avoid common phrases or words associated with spam mail, like "Earn", "Free", "Money", "$", etc.
About the Junk E-mail Filter - Outlook - Office.com
There are two parts to the Junk E-mail Filter: the Junk E-mail Filter Lists, and the state-of-the-art technology developed by Microsoft Research that evaluates whether an unread message should be treated as a junk e-mail message. This evaluation is based on several factors, including the time when the message was sent and the content of the message. The filter does not single out any particular sender or type of e-mail message. The filter is based on the content of the message in general and uses advanced analysis of the message structure to determine the probability that it is a junk e-mail message.