Discussion lists often require every message to be approved by a moderator before being sent to the rest of the subscribers (moderated lists), although higher-traffic lists typically only moderate messages from new subscribers. Companies sending out promotional newsletters have the option of working with whitelist mail distributors, which agree to standards and high fines from ISPs should any of the opt-in subscribers complain. In exchange for their compliance and agreement to prohibitive fines, the emails sent by whitelisted companies are not blocked by spam filters, which often can reroute these legitimate, non-spam emails.
A tracking pixel is a snippet of code that allows you to track your subscribers’ activity across various channels like your website, landing pages, and newsletters. Then, you also have the option to send emails triggered by certain behaviors. So for example, if a subscriber goes to your site and checks out your product page, a tracking pixel can send an email with a special offer for the specific product the viewed.