Hopefully this will help people who are looking for web based email for there users.
The cgi Perl script is called WebUMake Mail by Way to the Web Limited out of the UK.
They have just upgraded it to WebUMake Mail from MailNow. I've been using it for the last year as MailNow.
It lets you offer free email addresses to your web site customers.
All you need is a Catch-All email account for any domain that you have. It has just about everything you can think of.
The $300 US is definitely worth it! Check it out here: http://www.webumake.com/wmail/index.htm
I haven't had any complaints about the product. And very good support.

Sound interesting.

Can you tell me more about the risk of overloading the server. I heard a lot of problems generated by cgi scripts.

Can you recommend a server type ?

Can you recommend a maximum number of people using the service in the same time ?

I presently have about 150 users using the MailNow program. As for using the program at the same time, I could not tell you. Best if run on the UNIX platform. The script checks the email box every 15 minutes via a cron job.
And sends outgoing mail with sendmail.