We do something similar to that in our environment. We send our estatements via an encrypted/password protected pdf. To get the customer to prove they can open access the pdf we do the following steps.
1 customer signs form stating they have the minimum system requirements and want estatements.
2 we send an email to them with a sample statement attached in PDF format and ask that they reply to our email with the date on that sample statement.
3 if they reply correctly we turn on estatement and begin sending the statements in pdf format to their email address.
This sounds good for a "push" delivery system (and can be adapted for Mose's system.) The only thing I would change is the content of the customer's confirmation email message. In order to dot the I's and cross the T's, the customer's confirmation email to you should be an affirmative request or confirmation of the earlier affirmative request for e-delivery. Rather than asking the customer to use his/her email client to generate an email message, you're in better control if you use a "mailto:" link or some other way that you can control the Subject line ("Please sign me up for e-Statements") and or the Body of the message ("This is to confirm my request for e-statements. In order to demonstrate my ability to access these e-statements, I have opened and read the sample statement. The date on that statement is"......customer completes final part of message.)