Exchange Compatible Spam Filters?
I will second canit!
We have been using Canit for about a year (and MIMEDefang for four years before that) and it is freaking awesome. If pointy-clicky through a web interface is enough of a GUI, that is.
It is _highly_ configurable and super flexible. You can have one stream for the whole company's inbound mail, one stream per user, or use a user's attribute in AD (accessable through LDAP) to "dynamically" map their email to a stream. Cripe, you can plumb it any which way you want.
In our case, I have our CanIT host sitting in front of five mail servers (one exchange, one groupwise, one netmail, one linux/sendmail host for application generated email and one OS/400 mail - not domino) each hosting one or more domains, or different POs in the same domain and not quite 1000 users. To say the least, we are in the middle of some consolidation. CanIT is acting as the smarthost, and has been hucking packets between systems with nary a hitch over the past year and a bit.
You can tell canit to strip training links from other systems and stuff it's own in the boilerplate or in the headers. If you want, you can have it send a daily reminder if you have messages in the trap.
Since the time we have installed it, we have been hit with a Joe Job, hammered by random word spam, you name it. Our mail servers and our users see none of it. Between greylisting RPTN (think distributed spam tagging database) and a handful of custom rules, we dispose of somewhere over 99% of spam. 3 false positives over the past 6 months or so, and they were easily released from the trap.
And the hardware? We use a recycled three year old Poweredge 350. 1GB ram 1GB CPU. The only time there is a load above 0.02 on the machine is when it is merging the RPTN data.
Sorry if I am coming across as a fanboy, but damnit, I like this package. If you don't want to manage "another server" get the appliance an just use a web browser.
ObDisclaimer: I wrote the first spin on the SuSE rpms (spec file, that is) for RP based on their Redhat spec file.