If you are leaving on vacation or will be away from office for any reason, do not forget to redirect your E-Mail to another person who can deal with the E-Mails you received. Further, leave an Out of Office notification that will be sent as a reply to the E-Mail sender with details on who will be dealing with your E-Mails and when you are expected back. You may also give out the E-Mail and/ or telephone no. of the person handling your E-Mails.
If there is no one you can forward your E-Mails to, just mention in your Out of Office notification that you are out on vacation/ whatever reason. Mention the date you are expected to return. Also, customers would feel reassured if you tell them that you will contact them as soon as you are back.
Most importantly, do not neglect to deactivate the Out of Office notification as soon as you are back. People won't be impressed if they mail you and receive a reply saying the recipient will return on a date that has already passed.