One or more users of your domain are receiving unwanted mail from a external user id.
Option 1: Blacklist the id from sending mail to any user on your domain
Raise a ticket to blacklist the sender email id.
After blacklisting the sender email id, no users from the blacklisted domain will be able to send any mail to any user on your domain
Option 2: Ask the user on your domain to blacklist the sender using the End user Quarantine Console