a good anti spam for exchange 2003 ?

I highly recommend Symantec Multi-Tier Protection which includes antivirus and antispam both at the workstation level and the server.  It is the best antispam i've seen, catches almost everything and < 1/1million false positives.  There is also some firewall (which is not as good as the av/antispam IMO) protection with SEP (Endpoint Protection, primarily the workstation/server AV product.)

A Linux box as smart relay with SpamAssasin and gray listing.

MS Exchange 2003 sp2 has antispam filters bult-in. You don't need any third party antispam solution. To enable antispam filters go to Global Setting - Message Delivery. I sugest that you enable connection filtering at first. In Block List Service Configuration add: zen.spamhaus.org and cbl.abuseat.org. Block list will reduce spam for 90%. Then you enable Sender Id filter  (it will prevent sender domain spoofing).  In public DNS you must  enter spf record for your domain  (check with you DNS administrator or ISP). Also go to Recipient filtering and check if Filter recipient who are not in the Directory is selected. All configure Antispam filters must be enabled on Virtual server propertis - General - Advanced - Identification.  This is for beginning.
You could also enable Intelegent Message Filtering. If  your users use Outlook 2003/2007 they can use Junk Mail filter. Setting Store Junk E-mail configuration on Intelegent Message Filter will move message with SCL  8 or 9 to users Junk Mail. You could set tihis setting to 7.  I sugest you that you read more about Intelegent Message Filtering and SCL on Microsoft TechNet for proper configuration.  

Symantec's Brightmail solution is all you need, man I have it working in several places and it is great.  However it is not necessary to buy it separately anymore, *it's part of Symantec Multi-Tier now* you basically get it free with their enterprise AV solution.

Tarpitting and sender filtering are great I use those too.

@Datedman - Brightmail is a separate product and is sold as a separate product.  It is licensed per user and starts at GBP 16.08 per user - minimum of 5 users for just the AntiSpam Element and GBP 25.71 per user for AntiVirus and AntiSpam.  It also answers the question asked which is for an Anti-Spam solution.
The Multi-Tier Protection is Anti-Virus (Symantec Endpoint Protection) with Symantec Information Security for Microsoft Exchange which utilises Brightmail technology.  This is a packaged product and the cheapest way to buy it is for Small Business Server and it costs GBP 259.00 for a 5-user version and only comes in blocks of 5 minimum.
I agree that Multi-Tier protection is a good product and use it myself.

Okay thanks, I stand corrected--I guess I never buy it separately anymore because I use the multi-tier...and IMO it's much better to have it. :)  It would be pretty insane IMO to not have AV at the server level.  Also insane to me: using only MS's anti-spam solutions for 2003 (altho 2007 is another story.)

Totally agree with you on all your points but the question is about Anti-Spam - there may already be an Anti-Virus solution in place, hence the request for the Anti-Spam solution.
If neither are in place - then I would recomend an Anti-Virus and Anti-Spam solution and would recommend Multi-Tier as you would.
The suggestion to use the Microsoft's built-in solution is only as a starting point, I did not mean to suggest that it should be used as a total solution if that is how it comes across.  May as well use the built-in tools as well.

No i was talking about GorazdT's comment that you don't need anything but MS's built-in tools with 2003.  I prefer not to spend time deleting crap from my inbox if possible. ;)

Ah! - with you now and totally agree again.  I like a clean mailbox too.