"Suppose the emails contained evidence of some crime that was later committed?"
I'll see your straw man and raise you. Suppose they contained confidential company information that would affect share prices? Difficult to avoid suspicion, even if innocent, if there were then suspicious share trades.
The fact is that intra company email can contain all manner of confidential information. It could be customer or employee personal data. It could be product plans. It could be results of clinical trials. All sorts of things above a support desk pay grade and operation of the email system shouldn't depend on such an employee opening it to route it correctly. It should bounce and give the sender a chance to re-route it correctly, sight unseen by anyone else.