If all the machines are connected to the internet directly I would use the internet to route mail directly instead of hub spoke. Use SMTPAUTH or POP before SMTP to authenticate the users and allow them to email while remotely roaming on the internet (assuming no vpn).
naming convention is eg mailto:john.smith@sg.company.com, where sg is the location code - this is the way IBM does it and makes the most sense.
If the company has one main mail server (firewall etc) and the machines are connected via a vpn private network then HUB-SPOKE is fine..
Openacs makes sense for workflow and document sharing - for email I might use SquirrelMail (PHP based IMAP/POP) for webmail. It seems the most actively developed and works fine (I have also looked at IMP and others).
As far as mailer I recommend postfix(Exim is also excellent), it is a pleasure to work with. I don't like the qmail licensing or documentation (have a look at the code - almost no comments in it at all). But any of the mtas - EXIM, Postfix, Qmail or Sendmail would all do fine.
Email me if you have more questions,
Anthony
http://www.xminc.com/anthony/
