Hard to answer since you failed to post your Outlook version and the precise
method by which you are sending the emails.

In general, these prompts are from the Outlook Email Security Updates
contained in all current versions of Outlook and in all Service Packs after
SP1 for Outlook 2000. These updates add additional actions to the list of
those that are blocked from accessing the Outlook Address Book. Antivirus
and antispam add-ins are a common cause and others may be affected as well
(for example, Adobe Acrobat's Outlook addin called PDFMaker is a very common
one). As a result, one of the add-ins or programs you are using that access
your Outlook Address Book needs to be updated. Many add-ins were updated
following the release of Outlook 2003 and should work. If not you'll need to
contact the vendor for a newer version.

http://www.slipstick.com/outlook/ol2002sp3.htm#problems has a list of
applications known to trigger the security prompt.

You can find more information here as well:
http://www.slipstick.com/outlook/esecup.htm#autosec
http://support.microsoft.com/default.aspx?scid=kb;en-us;838871
http://www.outlookcode.com/d/sec.htm