Question about Outlook Express

I work from home and use my iMac as my desktop. My firm uses Outlook over an Exchange server since many of us are virtual. Whenever I have responded to and set permissions for calendar sharing with my colleagues, my Outlook freezes up frequently. Also, when I compose an email, I get my address in the sender slot along with those of my colleagues (though it is clear that I don't have permission to email from their accounts). Is there a patch for this to correct the bugs? Or do I have to forget about using calendar sharing on the Mac and get a PC?