I keep getting this message and the user name to whom it is sent does not exist. Is somebody using my mail server to send spam mails? How do I check this? And the other mail users keep getting lots of junk mail, how do I prevent this? I know that there is an option in ispconfig to block junk mails, and I have more than 200 addresses in it, is there another solution?
I guess this is bounce spam (someone sent spam with your email address from another server, and the bounces come back to you). There's nothing you can do. To block spam at MTA level, take a look here: http://www.howtoforge.com/block_spam_at_mta_level_postfix
