I've been using Outlook or Outlook Express since they came out. You can get Outlook as part of Office 2010. I have not used it thought, I'm new to Win7 this year.
Does Thunderbird have workgroups? I really liked that feature of OE since I belonged to an Autocad group and a couple of others. What other features does T'bird have that are simillar to OE. OE is hard to beat if it's used as a intigral part of MS Office