It's been awhile since I've used PEX, but my recollection is that every email includes instructions on how to unsubscribe. The opt-in form is something you can put on your website to add people to the mailing list, but if you think about it, not many people will be proactive and go to a website and look for a way to unsubscribe from a mailing list. They will just wait 'til they get an email they don't want, and then either "reply" to it and ask to be taken off, or follow the "unsubscribe" directions in the email.
I am a brand new PTO President. I am looking to use email more instead of only sending home paper notices and information in backpacks. The Parent Express Email program looks to be usefull. I noticed that there is an Opt-in form to collect email addresses. Should there also be an Opt-out form for those that wish to stop receiving PTO emails?