IMAP questions

[Sebastian Mindling]

Hi all, trolling for input from anyone with IMAP experience.

My company currently uses the ancient MSMail postoffice system and
Windows Messaging clients for internal email. (please keep the
snickering to a dull roar :) It has actually worked admirably for the
past six years, and provided all the functionality we needed with regard
to email groups, etc. for internal mail. For internet mail I run a
redhat-qmail server, and the WM client has a pop client service that
talks without too many hiccups. 

But now it's time to change. Good old WM has trouble with more and more
attachments, doesn't handle MIME well, among other issues. But the big
issue is external access to email. So I'm looking at IMAP, and wondering
what the group here recommends for servers and clients. Linux platform
is required, and open-source is preferable, although I won't mind paying
for the right package. We have <100 users. I will definitely need to set
up a web-based mail client (webmail?), and most likely will need client
software for internal use (on NT4 clients). Personally, I use Becky on
the PC side (http://rimarts.co.jp/becky) and it does IMAP. But I think
she's a bit much for general consumption here, and would like something
a bit simpler for the user. If I can find a good web-based client, I may
be able to forego the local client software altogether.

Any ideas/suggestions?
Thanks,
seb
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Sebastian Mindling