[Antivirus]

The only thing I can think of which may cause the userid to be missing is that you have another modification whcih is conflicting with this one.

Go to AdminCP->Plugins&Products->PluginsManager and look to see if there are any other plugins using hook location: mail_send. If there are, try changing the Execution order for the EZbounce plugin (at this same hook location) to be either higher or lower than the others. Test it by sending a test email to an address you know will bounce such as johuihiouyh98y9iuh96r855@hotmail.com then check the ezbounce header for a userid in the bounced email.

The only other thing i can think of is if you are allowing your users to use the "Send to Friend" feature which is naturally not going to append a userid since friends are usually not registered in the forums.