Spammer's habit of faking the sender address on their mailings with innocent email addresses, joe-jobbing spam is causing heavy server loads on company networks. The huge bounce or reply traffic for spam email is going back to innocent bystander networks and PCs.

Bounced spam messages hammer corporate networks | The Register

More than 50 per cent of the largest US corporations have experienced mail service outages or delays because of bounced spam messages targeting their networks, according to a study by gateway security firm Ironport Systems.

Spammers commonly forge the sender's email address so that some poor innocent - rather than a spammer - has to cope with the load of bounced messages sent to invalid email addresses. According to Ironport's study, these bounced messages make up 11 per cent of all all "hostile mail", other categories of which include spam, viruses, and phishing emails.

Spammers have been known to harass anti-spam campaigners by using their email addresses for the spam broadcasts. The anti-spam campaigners receive volume of bounce or complaint email. The practice has been ongoing for 5 years now, in spite of the CAN-SPAM legislation.