"market", used as a verb, means "to offer products/services for purchase", or "attempt to persuade people to buy a product/service".
(as a noun it means the place where this happens, or more generally the group of people who might possibly be interested in buying the product or service)
An "email marketer" is someone who is trying to sell something by sending out emails. They are not trying to sell emails - emails are their chosen method of marketing.
So, in your text, "People who send mass marketing email" (note this should be "emails" not "email" since it's plural) and "email marketers" are the same group of people.
The text also refers to "the people who market to email marketers". This refers to a different group of people who are trying to sell a product/service to the first group of people.
So, if you had some computers for sale, and you were emailing people saying "Hey, want to buy a computer?", you would be an "email marketer". If someone else then got in touch with you, and said "Hey, we think we could help you send more effective marketing emails, and therefore sell more computers, would you like to sign up to our email marketing system?" then that other person would be an example of "the people who market to email marketers". One of the things they might say would be "We think that email subject line length is very important, and we can help you come up with better subject lines", and that's what the text is referring to.