If you are going on a vacation and won't have access to your emails, it is a good practice to set out an auto responder. This way anyone trying to send you an email will get this response.
You can access Vacation Responder from Settings > Preferences > Vacation Responder.
You can set the 'from' and 'to' dates, a custom subject line, and the message you want included. If you have contacts sending multiple emails to you, you can control how frequently these contacts receive the auto-responder reply so as not to spam their inbox.