What is required to setup a low cost pop3/imap mail server

several issues that should be addressed.
all incoming mail will have to hit your internet connection on port 25 - thats just the way it works - but you could then port forward from your router to your mail server on almost any port that your mail server is then listening on - althought there isnt much to achieve by doing this - port 25 will be ok.

for a backup queue you need to have a second MX record for your domain dns with a lower priority.
This MX record should point to another mail server that will store and forward your email for your domain - hence forwarding it when your connection comes back up. Your ISP or DNS provider will often have this function.

For 40 mailboxes I would think you have plenty of other issues..how is your mail backed up? Are the users keeping mail on their workstations and how much mail will they get.

in many respects I would either look at paying your ISP to do this for you, or putting in Small Business Server to get Exchange at the best possible price. There is a little more to communications than just email. - Shared calenders, contacts, meetings, etc as well as backup, mobile phone access and web mail.

Good Luck

Thank you for your comments, but we are looking at using CentOS as the OS platform.  Our mail volume will be very small, maybe 3 or 4 messages to 50% of the users per day, and we have the diskspace and backup issues mostly addressed.  The secondary mail server with lower priority MX record is a good tip, but I'm not sure how to implement.  Any ideas on antispam/antivirus for the mail stores?  
Thanks

the mx needs to be configured with your dns provider.

cant help with the antispam/antivirus - I am using symantec for exchange, or Mailwasher for the odd home user.

Just what I was looking for.  

Thanks

Thanks again