1. Prospects have to give you permission to approach them. If not, you should not approach them as it can reduce your goodwill.
2. Permission is definitely selfish, realize that! Your prospect will mostly give you permit to approach them only if they are able to identify a reward or benefit in your offering.
3. Permission should be unsubscribed anytime the prospect wishes. You should provide them with the unsubscribe option but, if you have done your homework correctly there can be no going wrong.
4. Permission should not be transferred. You cannot exchange the permit with another person. It means that you only send an offer through e-mail to leads who really want you to send them though they have varying degrees of interest in your products. Maintaining the highest ethical standards in such a relation is very necessary.