The email industry would comprise a lot more than just marketing.
You have email providers. You have email clients. You have non commercial newsletters. You have evites. You have e-cards. You have emails related to ticketing and reservations.
The email industry is far larger than email marketing, despite what the email marketeers would like us to believe.
Either you do think email marketing comprises the entire industry, or the article title (and HN submission) are (or give the impression of) clickbait.
At least it doesn't seem you are doing any direct tracking on the post though ...
(I used to work for an ISP with a pretty large email service, so it touches nerves that were exposed when our enemies, those companies who try and find ways to irritate our customers, think they are the only important parts of the actual email industry)
This is something I wish ISP's took more care of, they could easily protect users by blocking image loading by default and warning users when links in emails are tracked, for example.