Since Microsoft Office 365 analyzes where an email is from as well as what the email contains, it is possible that something within the email could trigger a positive "junk" tag. 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)