Kim,
I don't think Thunderbird is designed for such really heavy use that you are talking about

It's a free client to replace crap like Microsoft Outlook Express. It does an excellent job doing that but I doubt it will be of interest to companies since there is no good calender, booking service etc. What is available now is not good enough?
Like you I replaced PM Mail/2 when it could not handle modern encryption, GnuPG, it made it impossible to use over different platforms. Also missing HTML was a drawback. I don't like or use HTML-mail myself but I receive a lot so it's very useful to be able to read it correctly. I have no hope to see PMMail/2 translated so I had to say goodby to it.

//bjorso