Of those, I'd say Postini is the best.
Google took Postini over a few years ago and since then the Google Mail junk email filter uses the same technology. It's very intelligent. If you configure everything correctly and report anything that gets through, it quickly learns patterns and trends as well as using its own logic and filtering mechanisms (reverse DNS lookup, SPF record checking, DNSBL, greylisting, etc). Obviously all of these features can be turned on or off at your leisure, as well as exceptions set for mail relayed through certain servers or from certain domains. You can even disable spam filtering for specific mailboxes within your organisation if you want - ideal for testing purposes.
You also have the option of a quarantine area where an administrator can manually log in and release / delete email that the system is unsure about, or you can set up functionality similar to MIMEsweeper's PMM where every user can log in and release / delete their own mail. Another benefit of this is that it keeps copies of all email a user sends and receives, and it can actually be used like webmail whereby you can compose email directly from a web browser and read your mailbox in a format similar to Google Mail. As for security, you have the option of multi-factor authentication by way of RSA SecurID tokens and a static PIN in addition to the traditional username and password method. It's also useful if a user deletes mail, since you can simply locate it within the Postini eScan interface and re-submit it to your mail server.
As I mentioned above, Postini are now owned by Google. Even before they were taken over, they were handling approximately 80% of the world's email every day and their infrastructure was second to none. It's even better recently - the speed is incredible (almost no latency when waiting for mail to arrive) and the web interface is quick to navigate and manage. It's built on the same principle as all Google's services - cloud computing - whereby the service is hosted and the workload distributed over a huge global network designed from the ground up for redundancy and performance.
Last time I checked it was quite expensive, but is definitely worth it. It's definitely worth the extra cost over MessageLabs.
In addition, it also has various reporting tools built in. You can do things like report on how big a user's mailbox is, how many emails users are sending and receiving, how much spam is getting blocked and the reasons why, see network outages, see the path your email will take to reach a given mail server (kind of like a visual traceroute), see which domains send you the most mail and even see which countries your mail is coming from. All in all, it's a very good service and is well worth the money.
BTW, I'm not a salesman lol. I'm just speaking from experience because I really get on with the service.