Sadly, I in many ways agree with Jerger on Smartertool's "selective" support.
Often times, Smartertools is exactly what they claim to be in their replies, attentive and responsive. However, at the same time, if an issue arises with their software that they do not want to address and deal with, they will simply ignore ignore the poster's request and in my case denied an issue even existed, even with their "paid" support services. The 100% CPU usage issue is definitely one of those issues and it has been raised several times by several different people and I always manage to add my 2 cents in the thread because of the way it has been blatantly ignored by Smartertools.
The most common cause of the 100% CPU issue is with the ClamAV process on 64 bit windows. You never mentioned which version of Smartermail which version of windows you were running, however, another customer also had reported a CPU issue in this thread http://www.smartertools.com/forums/p/16110/45622.aspx#45622, that was never really resolved.
The ClamAV problem has been posted about and discussed in several places including (several are me begging for a fix):
http://www.smartertools.com/forums/p/18371/47151.aspx#47151
http://www.smartertools.com/forums/p/18246/46729.aspx#46729
http://www.smartertools.com/forums/t/18948.aspx
http://www.smartertools.com/forums/t/17519.aspx?PageIndex=1
The final thread is the loggest running and most detailed on the issue and includes a workaround (assuming it is ClamAV pegged at 100%). Also evident in the thread is Smartertools lack of response to the issue until someone chimes in to praise their support after I bashed it only to finally invite a response from Smartertools that simply patted themselves on the back for doing nothing.