Exchange replacements

[Patti Ames]

I'm looking at enterprise-grade email/calendar servers that work well
with Outlook. I tried Scalix - if everything else is like that, I'm
going to find myself running Exchange pretty soon. :\

What have you all used to satisfy management's need to calendar and
reserve conference rooms inside of Outlook? Since my userbase are 
already pretty solidly occupying PSTs, easy migration is key. (Scalix
crashed Outlook or popped up virus warnings on attempts to merge old
contacts and calendar with the Scalix one.)

The other half of my users use Thunderbird, so it would be great if 
they could, in some way, also schedule and find out about meetings and 
conference room availability.