No, it is not always true, and thier can be many instances of False Positive.Items like newsletters or magazines that you have subscribed to are more likely to land up in your junk folder. If you are certain that a message in the Junk Email folder is something that you really do want, you can move it back to your Inbox by right clicking on the email and selecting mark as not junk.
Some of the most common reasons for an email to be marked SPAM are:
- The email was sent from blacklisted IP or domain.
- Sender's domain has "not" or "mis" configured SPF/DKIM record.
- There are several other reasons apart from the above mentioned, for a email to be marked as SPAM.
In case of a legitimate email that is marked SPAM,
- You can move the email back to your inbox;
- You can report it as not junk (right click on that email and mark as not junk)
- Add that sender to a safe senders list or a white list by going to Setting->Mail->Junk email or use the link (https://outlook.office.com/mail/options/mail/junkEmail)