Email open rate is the percentage of emails opened by the recipients out of the total number of emails sent by you.  These emails can be mere messages, newsletters or promotional emails.  You cannot expect every recipient to open your email message.  Normally only a small percentage of emails are opened every time you send them.  An average percentage of 15 is considered to be good when it comes to open rate per email message sent to recipients.  Emails sent with the help of an email marketing software can get you healthy email open rates.