On Fri, 14 Jul 2006, John Kelly wrote:
Every day, I get mail delivery attempts to non-existent users like: k2159jcd003343@isp2dial.com k1mmcsoa007563@isp2dial.com k1nardpb001747@isp2dial.com
This is a Joe Job. http://en.wikipedia.org/wiki/Joe_job A spammer is using your domain with the random usernames to send spam to other domains. These likely are the bounce messages from domains that queue SPAM before rejecting and don't check SPF records.
Since you have a SPF Record for your domain, Not much you can do except not accept them as pointed out earlier.
Steve