Are you really willing to risk not only your email deliverability, but also the reputation of your IP address and your company? Even if you find the light after purchasing or renting email lists and decide to only email those who have opted in with your company, it'll take you months (or maybe years) to get your Sender Score up and rebuild the reputation of your IP.
A web-based interface is often available to allow people to subscribe, unsubscribe, and change their preferences. However, mailing list servers existed long before the World Wide Web, so most also accept commands over email to a special email address. This allows subscribers (or those who want to be subscribers) to perform such tasks as subscribing and unsubscribing, temporarily halting the sending of messages to them, or changing available preferences - all via email. The common format for sending these commands is to send an email that contains simply the command followed by the name of the electronic mailing list the command pertains to. Examples: subscribe anylist or subscribe anylist John Doe.
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.