Hello,
How many users does the system need to support?
How many domains does the system need to support?
How much disk space will each user be allowed to have?
What mail systems do you already have experience supporting?
I have created and supported several small/medium size
netqmail /
vpopmail /
horde+imp based solutions over the years.
That combination seems to cover much of what you are looking for.
A variation of it might also satisfy the LDAP requirement. Here is a link to
qmail-ldap
Does this need to be something that you support on site?
Is outsourcing the mail hosting an option?
to Email Questions!