Sandeep has posted a blog about Request Handler Installation problems

http://www.duet.com/Default.aspx?tabid=70&EntryID=68

I would just add that an additional thing to check is that the user you are entering has an SMTP address (whether primary or secondary) that matches the domain for the Exchange virtual directory in IIS. So if your Exchange virtual directory points to \\.\backofficestorage\mydomain.com\MBX and you enter yourdomain\RH_Account as the user in the installer, ensure that you have an SMTP address for RH_Account that looks like RH_Account@mydomain.com in the list of email addresses for that user in ADUC.