Version of Outlook?
Type of mail account?
Do you have a virus scanner installed which integrates itself with Outlook?
Uninstall this integration part of your virus scanner and try again; you would
still be sufficiently protected by your on-access scanner part of the virus
scanner. For more details see;
Robert Sparnaaij [MVP-Outlook]
Coauthor, Configuring Microsoft Outlook 2003
Outlook FAQ, HowTo, Downloads, Add-Ins and more
Real World Questions, Real World Answers