Duplicates functionality already built into Thunderbird
I downloaded and installed the Cloudmark Desktop for Thunderbird on Windows XP. The results using different email clients or different operating systems may be different from what I found with running it on my system.
What I did find was that Cloudmark initially misidentified some of the legitimate emails that I received as being spam. This shows that it needs a decent amount of training up front before it can correctly identify what is and isn't spam. Fair enough, Thunderbird itself required quite a bit of training when I first set it up. The difference though is that Thunderbird has never misidentified a legitimate email as spam.
I examined the files that Cloudmark had identified as spam and ran the Thunderbird builtin spam filter on that folder. In every case every email in the folder that Cloudmark had correctly identified as spam was also identified as such by the built in spam filter in Thunderbird itself. The only emails that ended up in the spam folder after that were the ones that Cloudmark had misidentified which it should have left in my inbox.
Possibly with sufficient training Cloudmark would become as good at identifying spam as the built in spam filter in Thunderbird but there is no point whatever in running Cloudmark in Thunderbird since the built in spam filtering does the job of filtering the emails. All Cloudmark is doing (assuming you spent sufficient time training it to get rid of the false positives) is adding an extra step that simply duplicates the built in spam filtering without actually flagging the emails as spam.
This antispam filter may be useful in other email clients that lack the inbuilt spam filtering capability of Thunderbird but Thunderbird users do not need it at all.