I host websites for the websites that I develop for clients. Currently, I have about 10 websites and domains that I host. I have about 30 – 40 users that use e-mail off my server. Right now I'm using Sendmail for my MTA which uses virtusertable so that I can set up unique users for each domain name. I then use Procmail and mail local for the MDA. For IMAP and Pop3 I used imap-2001a-10. I mainly went with this setup because this was what Red Hat came with when I started learning about setting up a mail and web server.

Here is my question... As my business grows I would like to add mail users with out having to add them as a system user. As it is now Sendmail's virtusertable maps mail to system user account or another external e-mail account. Also as it is now I can have two people with the email addresses user@domain1.com and user@domain2.com but I would have to map those email addresses to separate system accounts like user1 and user2. And that makes things confusing for EU's when you tell them their email address is user@domain1.com but their account name is user1. I've used ISP's that have have you use your full email address for your account name when using pop3 or IMAP which leads me to believe that you could have user@domain1.com and user@domain2.com with out having to have separate unique system accounts. I would like to have a setup where I could do the same thing.

On top of all that I would like to add new mail users by using a web GUI. I also would like to control the mail box size for each user. And I would like to monitor bandwidth and storage for billing reasons for each domain name.

Does anyone have an Open Source solution they are using right now that does what I want and that isn't that hard to migrate from my existing setup?