It is all in how you ask your customers for their email address. My kids elementary schools sent out a green piece of paper that said “We’re Going Green” (the paper wasn’t even cut properly). They didn’t ask families, they told them. They even told me to look in my junk folder and make sure I mark them as “safe”. Position it with your customers the same way; as an expectation, not a request. “I expect you to do your part in my going green.”