Marketing Newb: How to prevent "over marketing" to customers?

We have developed a new "Friends and Family" referral program and I would like to send an email to all existing customers that meet certain criteria.  This initial campaign will go to a large number of customers.  We will want to do this again in a month or so to get the same offer to new customers and any customers that we have since acquired an email address for.  We have regular contact with our customers and most don't have email, so it's not uncommon for us to finally get an email address for a customer we've had for years.