you might just not noticed but i would almost bet that it did happened to you as well. we noticed it after 1.5years of using mandrill. and only because one our user contacted us directly and was patient to cooperate with tracking the problem. after that we discovered plenty other cases. and communicating with mandrill revealed that it is how their system in fact works. anyways if you are happy with Mandrill good for you, but remember my warning: you are living in a dream that all the emails marked by mandrill as delivered are in fact delivered.