Recently, I noticed that I was receiving Returned Mail messages from
various mail servers, saying that "host not found" or something like
that. This is not unusual, because of spammers using spoofed E-Mail
addresses on my domain.

What I found odd was that the message that was sent that caused the
"not found" error was a read receipt from Microsoft Outlook 2007
(which I use).

To test this finding, I sent myself a message with a read receipt
attached to it, but I didn't read the message and deleted the message
(which put the mail message in my Deleted Items folder).

When that message was cleared from the Deleted Items folder, a "not
read" receipt was sent back, even though I have Outlook set to prompt
me before sending read receipts.

This strikes me as an effective way for spammers to gather valid E-
Mail addresses even when Outlook's spam filter catches their messages.
The "not read" receipt is being sent, even though I have told Outlook
not to, which in my mind, constitutes a risk that Outlook shouldn't be
exposing me to.

Am I off-base with this? Is there a way to turn off the "not read"
receipt function that I'm not aware of?

I did a search, and found a similar bug report from 2003, and it
strikes me as odd that something like this wouldn't be fixed in four
years.

BigScreen.com wrote:

When that message was cleared from the Deleted Items folder, a "not
read" receipt was sent back, even though I have Outlook set to prompt
me before sending read receipts.

This strikes me as an effective way for spammers to gather valid E-
Mail addresses even when Outlook's spam filter catches their messages.

In general, since spammers send their junk from bogus addresses, there's no
valid address to which to send the receipt, so no harvesting would happen.
--
Brian Tillman [MVP-Outlook]