Recently I'm receiving frequent messages along the lines of
did not reach the following recipient(s):
" You do not have permission to send to this recipient. For assistance, contact your system administrator."
I've run all the normal packages to ensure no viral or spyware infections on my PC, and im behind a NAT router.
Is there any way to be certain that these messages are just down to my address being spoofed rather my PC actually being used to send mails ?