Spamassassin is slow - any tips or good commercial alternative?

[Charlie]

>> The concern is that I am eventually looking to have over 10,000
>> users, so will be receiving, and then sending, multiple emails per
>> second.
>> Even now, with only 1,500 users, people have started reporting "Too
> many concurrent SMTP connections; Please try again later"

>I manage a mail cluster for an ISP that has about 100,000 subscribers
>online at any given time.  I'd hazard a guess that close to all of
>those subscribers have a mail client running the whole time.  We have
>implemented an exim set up that allows 3 concurrent connections from
>the subscriber IP pools, 5 from from other IP's and a maximum number
>of simultaneous SMTP sessions of 1000 per server.  This is on each
>server in the cluster (4 servers in total) and we only ever see the
>maximum concurrent per IP rules being hit, never the maximum total.

>As you didn't mention which MTA you're using, I can't give config
>examples, but make sure you pay close attention to the default max-per-
>IP and max-total SMTP session limits.  If these are still the
>defaults, they are probably far too restrictive for a multi-thousand
>user mail hub.  Also, if you're not already, put some thought into
>connection mitigation techniques such as RBL's, grey listing, etc.
>These will have a dramatic effect on external sites chewing up all
>your SMTP connections.  Also if your users are the sort of people who
>sit there smashing the "send and receive" button in their mail client,
>you may also want to think about rate-limiting connections from any
>single IP...be careful if you implement this, you can DoS your users
>if it isn't done right!!

Thank you James - this will be very helpful I suspect.
We also are using Exim as the MTA, so any specific config advice for Exim 
would also be greatly appreciated :)
BTW we are using one server (Pentium 4, 2.4GHz, 1GB RAM), currently have 
1500 active paid users,
 and am expecting up to 10,000-15,000 active paid users in the future (say, 
1 or 2 years from now).