Recently, there have been a lot of unsolicited bounces from ISPs which are created due to the following chain of events:
- Spammer uses an infected computer on an ISP network (zombie) to send spam
- Spam is sent with a forged sender address (aka. From), that is not hosted by the users ISP.
- Spam is rejected during delivery to recipient mail server and the receiving ISP mail server generates a bounce to the original, forged sender.
Here are some possible solutions to this problem (your ISP may need to be involved to manage these recomendations):
- Use some form of authentication on your domain; SPF, Sender ID or Domain Keys Identified Mail.
While these solutions will not solve the bounces coming into your network, but will help ISPs checking messages supporting these solutions determine if the original messages that they are receiving (and subsequently sending a bounce to you) are from your network.
- Implement a solution like BATV for your own messages, to help determine legitimate bounces from your network, this allows you to ignore all bounces received that do not comply with your configuration.