If Outlook is not configured to request a read receipt for your
outbound e-mails then maybe something upstream is adding those headers
to your outbound e-mails.  Since you are posting through Microsoft's
NNTP server, and since Microsoft bastardizes the NNTP-Posting-Host
header (to point at their own host rather than the poster's host), I
can't tell which ISP you are using.  Maybe there is a server-side
option on your mailbox that lets you enable read receipts.  I haven't
seen this but it's possible.  Use the webmail interface to your e-mail
account and check its options to see if any could alter your outbound
e-mails through that account.

Have you yet tried running Outlook in its safe mode ("outlook.exe
/safe")?  That will load Outlook with any plug-ins.  Perhaps you
installed an plug-in, like for anti-virus, anti-spam, anti-malware, or
whatever, and it is doing this automatic header insertion (read and
delivery receipt requests are defined by adding a header to your
outbound e-mail that the receiving e-mail client for the recipient
needs to understand to see the request).

In the Sent Items folder, use the View -> Options menu to look at the
headers for the problematic e-mail (the one the recipients say is
requesting a read receipt).  Check if any of the following headers are
in that copy of your sent e-mail (see
http://www.ninebynine.org/IETF/Messaging/draft-klyne-hdrreg-mail-00.html
for short descriptions):

   Read-Receipt-To
   Return-Receipt-To
   Return-Receipt-Requested
   Disposition-Notification-To
   Generate-Delivery-Report