It seems some spammers have been using the init1.nl domain to forge from-addresses in their spams. First of all, my server is not being used for this, they are all using open relays/proxies, this is out of my hands.
I have however chosen to add an SPF TXT record to my domain:
init1.nl. IN TXT "v=spf1 mx a:rudeboy.init1.nl -all"
This might not prevent the spam from reaching your mailboxes at this time, but if you use SpamAssassin 3.0 or up, the mails will be scored with extra points because they originate from unauthorised servers. As soon as more providers start implementing SPF the amount of spam with forged from-addresses (and with that the bounced emails) will hopefully drop. An easy wizard for creating an SPF record can be found at http://spf.pobox.com/wizard.html.
