What is backscattered email spam?
In Internet, the sender address of an email is forge-able, someone pretend you to send an email to a 3rd party SMTP server.
If the 3rd party server accepted that email, but found no such user, and bounced that email back. That email will be bounced to the forged sender email address.
The (true) sender email address received that bounce. If the bounced amount is great, it is back-scattered spam.
(The situation is similar to send a letter in General Post Office. The envelope sender’s return address can be forged. After the mail is sent, if the postman fails to deliver the item to the recipient (eg: no such person), the letter will be returned to the sender address. The difference is that the postal delivery requires postage fee)
For further information, you may refer to https://en.wikipedia.org/wiki/Backscatter_(email)
It is difficult to filter back-scattered spam, as those emails are bounced from legitimate email server (with room-for-improvement configuration)
If the volume is great, users may consider to use a Outlook rule and temporarily filter those bounced back messages.
Note: it will move all bounced back message to a selected temporary folder, no matter it is a back-scattered spam or a real bounce.
Inside Outlook, goto “Rules and Alerts”
Choose – apply rule on messages I receive
With specific words in the message header
Enter the specific word – Return-path: <>
Move the message to a specific folder
Give a name to the rule.
Those new bounced back message will be moved to a specific folder.
User may regularly check the bounce folder for any ‘real’ bounce.
If the problem disappeared (normally shall be last for a few days), you may consider to remove the rule.