I was once a sysadmin for a small ISP .
First, I don't think e-smith has been tested for multiple dialin and there are proven and fairly inexpensive ways to do it using 'digital' modems. You really don't want analog modems answering at the ISP if you can avoid it and you should if you have 16 lines or more. It is a real time sink to keep modems running and realize that in a typical hunt group the 'bad' modem will get hit by every user that logs in. The only way more users get to log in is if two users dial in near enough to the same time that one of them gets assigned a 'good' modem. I had to repower modems every night to improve reliablility.
We had a separate Radius access server running on a Sparc 2 and several Lucent Portmaster 3 routers connecting 250 digital 'modems' through a T1 to make the dialup connection.
However there are other ISP things e-smith should do well. We ran 'Slowlaris' on a Sparc 5 (55mhz) with 128M and 10g of assorted SCSI drives to support primary DNS and about 3000 email accounts and 250 virtual domains served by apache. The Sparc 5 was the most reliable service component. It did great, except that due to the homegrown GUI tool to add users, it became irritating. It took well over a minute (try waiting a minute for each entry to see how much fun it is to add 5 users) to add or delete each user after about 2500 users. To avoid web server slowness complaints my guideline for CPU utilization was to watch out when the load average went over 2%. I once saw it as high as 90% when I had occasional (half a dozen in 3 years) denial of service attacks.
The person that built the ISP system claimed the Sparc 5 was equivalent to a 233mhz Pentium for performance. The only extended outages were a result of disk failures about once a year.
E-smith has many more features and benefits (a more sophisticated template system) than what I used to add virtual domains. The backup software I used was pretty primitive and as a result, not very reliable. We could not guarantee backups over the weekend.
With good hardware (fans, power supplies) I would use e-smith 4.0.1 for web & e-mail service without a second thought. DNS of course is another matter and e-smith has discovered that ISPs often break their DNS servers through mergers and aquisitions; and avoid the problem entirely by referencing root nameservers only. That is, e-smith wisely recommends you don't specify a name server! My thought is you have to have be very careful and consistent to maintain a reliable DNS service as a ISP.
And last, when 4.1 is released I would take a look at 3Ware's packet switch based ATA disk controller which I believe has been tested with e-smith and could cut drive costs in half.
IMHO you should be able to serve thousands of accounts with e-smith on a Category IV server. The cost of hardware and software is vanishingly small. Labor costs however, remain high:-)
Hope this helps.
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