Email can be admissible in court, although getting it admitted can be problematic depending upon the judge. Even when admitted as evidence, the amount of weight given to the email might not be as great as you expect. Email evidence is becoming more common in the courtroom, however, and should be used if it is available and supportive to your case. This article has more information on this subject: Using E-Mail as Evidence
Voicemail and answering machine messages are nearly always admissible because the calling party knows they are being recorded. Recorded messages also seem to be more readily accepted by the court than email traffic.