If you start receiving an increased amount of spam, and you have your Junk Mail filters enabled, there could be a few reasons why this is happening and ways to fix it.
New Spam Techniques
It may simply be a spike in spam emails being sent across the world, or a new spam writing or format that is evading the rules we have in place for Junk Mail. Keep an eye on the number of spam emails you receive, and see if it goes down naturally.
Full or Disabled Mailbox
If you have set up your Junk Mail filters to redirect to a different mailbox, please check and see if that mailbox is full or disabled. Once email can no longer be sent to that mailbox, it will go to the default mailbox.
Bounce-Back Emails
If you are receiving emails that are bouncing back from recipients and look like they are spam, it is plausible that your email address has been spoofed, your mailbox has been accessed, or your computer has been accessed. While there is very little you can do if your email address has been spoofed, you should run a full antivirus or malware scan on all computers used to access your mailbox, then change your passwords to strong and unique passwords.