Spam filter products for service provider

Hi,

We are a small service provider and currently we are using Symantec's Mail Security (Windows2003 server) to filter spam. We are offering this as a service to our customers. There are about 50 domains that we handle with this software.
However Symantec's software support will end soon as this software goes EOL.

Do you have any suggestions how to replace this product ? I would like to see few features in that software:
- supports Linux ( or Windows)
- just software not an appliance aka machine
- "a whole package" as we don't have time to build it ourself.
- good support from manufacturer
- commercial

Thanks,

Jysse

If you run Linux as your mail server, spambouncer is my
favorite. I don't know what to run on a windows server.

The SpamBouncer: a Procmail-Based Spam Filter