Securing sending postfix e-mail

I suspect that you are confused as to the real source of those emails and that you are taking what's in the From: header too litteraly. The From: header corresponds exactly to what's on the piece of paper inside of an envelope. It can be completely different from the envelope Sender, in the same manner that what's on the paper inside of an envelope isn't seen by the postoffice. The From: header can contain anything, or even be absent and the mail will still be delivered to the envelope Recipient.

To see where the message really came from you need to look at the envelope headers and how that's done depends on what mail client you use. For example with Evolution you select "View->Message Display->Show Full Headers". With that visible the first "Received:" from the bottom is the system that originated the message.

If you want to watch this traffic in real time, I suggest getting ethereal (or tethereal -- the text based version for the command line) and use the following filter: "tcp port 25", which will catch all SMTP mail traffic.  Watch this traffic for spoofed mail ... in doing so you may be able to determine also if mail is being relayed from your box or if the offending mail servers are enumerating mail accounts using the VRFY command (it's basically like saying "Is Joe a user on this mail server?").

Make sure the following line is in your postfix config file:

disable_vrfy_command = yes

-tcmv
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