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.
• Email content may contain "Image links to remote sites", "Numeric IP address in URL", "URL redirect to other port", "Empty messages", "JavaScript or VBScript in HTML", "Frame or IFrame tags in HTML", "Some sensitive Word" etc. etc.
• 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)