This service has been clearly aimed at large (or complicated) organizations that might be managing multiple domains. It can do clever tricks such as rewriting email addresses and handling subdomains that may not have been specifically configured. Administrators can be created with limited sets of responsibilities.
The interface is well designed, but the opening dashboard displays global statistics, rather than those pertaining to the client's domain, which we would have preferred. Configuration is hirearchical: each customer account controls any of several organizations, which in turn manage any number of domains, which then include usergroups and individual accounts. With that degree of granularity, we expected rule inheritance to be complicated, but the firm has done a good job of making it all sensible.
The service can run in an observation-only mode, tagging headers onto suspect spam but otherwise leaving them alone, which is great for testing.
The option to integrate with an LDAP directory was disabled at the time we tested, leaving only the facility to upload a list of users in CSV format. If you do not, then the system assumes every incoming user is valid. A clever touch is the option to forward rejected emails to the last entry in the list, which can then be used to catch exceptions.
Quarantine options include "SpamShark", a webmail quarantine that is created on the fly for incoming recipients of junk mail.
Our only complaint is with virus handling, which by default sends notification to the sender, if it comes from outside, but not to the sender if it is internal. Given that nearly all email viruses spoofed the address, it would seem more sensible to reverse that and notify the administrator of a potential internal infection, but silently delete viruses coming in from outside.
The documentation is excellent, and the online help very good. Reporting is well constructed, with a lot of data and useful charts, but it has limited filtering options and the actual reports take a while to run.
Frontbridge is a strong contender, especially for large, complex environments.