This is spam.
Basically some spammer maybe sending email using your domain name even though its not really coming for there. there faked headers spammers use.
So your the one stuck with the reply because you got a catch all email box that just grabs all email like a *@mydomain.com
The Bounce Email option is not available in ISPconfig so just disable the Catch ALL email options for each email and you wont get this.
Also make sure you spam filter is on.
One day i hope to see the bounce email feature it really rocks.