Do you avoid using words that make your email like junk mail?
Outlook by default reads your emails and can flag your email as potential junk email, or adult content email by examining a list of Sensitive words. This list is not available to the public.
You should avoid any swear words or be using an angry tone as well.
We have a program called SSW LookOut! for Outlook to check for this rule. Figure: SSW LookOut! for Outlook checks that you don't use bad words.