Returning too much undelivered emails to us. But not ours

Hello,

As you know spammers use fake emails to send bulk of things to thousands. One of them is always using our company email as a sender, so all of undelivered emails are returning to us. They are not sending those emails from our server, it isn't a relay server, using somebody elses relay server but using our email address as sender.

yesterday I cleaned more than 20.000 emails from Postfix server and this happens very often. :(

I'm using Postfix 2.2.10 + Spamassassin + Mailscanner on CentOS 4.7 (rhel4)

Any help? How can I filter those unwanted returning emails? :(

Thanks in advance

hey,

Few options I know are
1. activate SPF records -
2. set mails to fail or black hole
3. go for third party software such that mails are scaned on the gateway..