Message allocation beetween Inbox and Junk

I'm new to outlook.com.  Two related problems:  (a) The system keeps sending wanted messages from trusted sources (even calendar reminders!) straight to my junk box.  Marking such messages as 'not junk' does not stop the same thing happening next time round.   (b)  Some of the messages that do arrive in my inbox are junk and I mark them as such.  However, the next time such a message comes from the same source it aqain goes into my inbox instead of being automatically routed into 'junk'.  How can I correct these?