I'm sure this isn't helping now - two years after you asked your question - but it is possible for customers to update their own email addresses. There's an option at the bottom of each email you send that allows customers (or church members) to make changes to their own profile. The problem is, I don't think you, as the administrator, get any notification when an intentional change has been made, and I haven't found a way to pull a report on emails that have been changed by customers. It's possible customers made errors when they entered new information on your account, and that kept them from getting emails. So, people with access to the account are not the only ones who can edit your list, and I'm very surprised Caitlin_M didn't mention this possibility. You can actually create new problems by reverting updated emails to the old ones when you re-do a list. bummer...
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