I'm going to say that it really matters who the email is from. If it is from someone within your own company, and not the highest level of management, then a quick reply that you think they have the wrong person, what you do, such as geekrunnings' example shows, is very appropriate.
If the question is from a high level of management, it's worth asking your manager for who might be the right person or department, and send an apology email back, but cc that person as well. That gets it going in the right direction and shows that you're not just blowing them off.
I think this was sent to the wrong person, but I've been told that Justin Thyme in Shipping might be the correct person to handle this. I'm sending this request to him, but please let me know if I do need to help in some way.
If the question is from outside the company, then you again want to track down who might be the right person, starting again by checking with your manager. You want your company to look like it cares for its customers, so it's important to care about getting the email to the right person or department. The reply is similar to the reply to management, but show that you're willing to be contacted again if this redirection doesn't help.
I think this was sent to the wrong person, but I've been told that Justin Thyme in Shipping might be the correct person to handle this. I'm sending this request to him, and here is his contact information as well. (include phone and email for contact as appropriate.) If your request isn't handled, please let me know, and I'll try to find out more how we can help you.
If your manager also doesn't know who might be the correct person to contact, then it's worth looking up in a directory and trying to find someone. Or asking the manager who might know. For a CEO or customer, you really do want to direct the email in the right direction as well as sending a reply back. If you really can't, then at least cc your manager in the reply back, so the responsibility for finding the right contact is moving up the chain.