Junk email filtering in Office 365
If you're a user of Office 365 with a hosted mailbox, there may be times when a message ends up in your inbox despite the fact that it was marked as spam. When this occurs, it may be because you have (somehow) disabled junk mail filtering. When this occurs, email is still marked as spam, but instead of going to your Junk Email folder, it lands in your inbox.
How to fix this if you are a regular user
There are multiple reasons why this could occur, but the most common is that the setting that moves your spam to Junk Email has been disabled. To fix this: Settings/Options > Mail > Mail > Accounts > Block or allow > Automatically filter junk email
In most cases, this will resolve the problem.
How to fix this if you are an administrator
As an administrator, you can also see if the user has mailbox filtering enabled using Power shell. It's pretty easy:
$junk = Get-MailboxJunkEmailConfiguration -Identity $email
$junk | fl Enabled
Enabled : False
You can then run Power shell to change this:
Set-MailboxJunkEmailConfiguration -Identity $email -Enabled $true
That will flip the user's setting to move spam to the Junk Email folder.