A recently published article by WPBeginner explains how to export email address from the comments and import them into a mailing list. While the article advises getting the user’s permission, everything about this practice rubs me the wrong way. If you’re going to do this, please put a big banner near the comments that states your intentions. A quick and easy way to do this is to use the Show Comment Policy plugin by Jimmy Peña. The comment policy text will be displayed above the commenting area.
I’m most concerned about sites that export email addresses from existing comments. At least give those people the common courtesy of being notified and provide them the option to opt-out immediately. In fact, it may be against the law in certain countries to outright harvest the email addresses. Better yet, instead of getting involved with this practice, just turn commenting off on the site.
Are you ok with the email address you gave to a site in order to leave a comment, being added to an email list as long as you’re notified?