I tell my players I back them if there are problems – and I do.
If you are going to do this, I would make it a choice that does not use a pop.
Try usine the signup mailermailer provides – it’s free for the first 200 signups and it’s a page you can customise with lots of features. They automatically do an optin message, or double optin as you like.
I have done this for years and it works well for me
Mailermailer has superb tracking with all kinds of drill downs, so at least your clicks through your letter will be tracked perfectly.
Hope that helps re. the signing up members…