The ubiquitous nature of E-mail makes it a simple and useful means offeedback — every CI server I'm familiar with provides some type of E-mail feedbackmechanism.
In the case of CI, this action must be prompt because a broken build affects everyone; therefore, the feedbackmechanism employed by a CI server must be timely.
In my opinion, rapid feedbackof build events is the central tenet of CI and one that causes many to describe a CI system as a continuous feedbackmechanism.