I run qmail and horde/IMP on a RedHat box.  Works great.
Your hardware should be just fine for 20 users, even if they are very active. 
What type of connection are you planning on using?

Quoting John Meier <john.meier at gmail.com>:

> I have about 20 folks who'd like email addresses for their domain and
> a web based email client interface is all they really want.
> 
> any recommendations? 
> 
>  It'll run on an old dell 4200 server which has dual P2, hw RAID w 5
> scsi drives 512 RAM.  I have fedora up and running and am poised for
> installing and configuring an email system.   I'm hoping this will be
> enough power ...
> 
> volume will be low, bu I don't know exact numbers...
> 
> thanks
> 
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