The way most people solve this is with a unique code attached to some tracking pixel that gets picked up when the HTML email is opened. We use an alternate of embedding the unique code in the links for the email so that even a text only email will trigger a click for that user if they copy/paste the link. You lose "open" tracking, but this isn't too bad unless you are selling ad impressions for your emails.
And how many email users load images by default? How many of Mozilla's more technical demographic?
If I legitimately want emails from you (Mozilla seems likely) and you stop sending them, I'm going to be irritated. But if you stop because your phone home 1x1.gif doesn't load, that doesn't mean I wasn't reading them.
At least for my site, we consider multiple channels of engagement when calculating whether a user who is subscribed to something is active enough to bother messaging. If you don't click/open the emails (as far as our tracking can tell), but your account still opens our app, you'll keep on getting email updates until you unsubscribe via the unsub link, the profile preferences, or you close your account.
Mozilla is probably in a bit weirder position because their value-prop isn't in curating articles and videos the user is interested in. They don't require the users to login to use Firefox, and thus can't know if text-only users are using their emails unless they process unique ids on links inside the email (and that's the last step of the usual funnel, which means the segment definition is going to have to be really long (probably a year)).
If you don't copy/paste a link from any email within a year you end up falling off the "active users" segment and we stop wasting our time sending you emails. For text-only users (which we seem to have a few who have "clicks" but no "opens"). The particular system I work with is actually slightly more sophisticated and considers usage from a lot of channels (push message consumption, email opens and clicks, app usage, alexa usage, bot interactions, and website usage). If you don't do any of those things you fall into less and less frequent push/email interactions until we stop bothering with routine newsletter/updates messages except for an occasional "please come back"-style campaign.
"Copy/paste" here means "Copy the URL, paste it into the address field of a Web user-agent program, and initiate navigation to that URL in that Web user-agent program".
I suspect the phrasing was used since the discussion is about text-only email and, this being Hacker News, someone might have nit-picked that they view plaintext emails in a mail user agent program which does not scan for strings matching the format of a URL and offer a direct way to click on them or otherwise trigger automated opening of them in a Web user-agent program, and so the person who wrote "copy/paste" had to avoid saying "click on the link".
Copy pasting into the browser to view the link triggers this, just copying doesn't. If I understood the previous comment correctly, the author copies links into the browser instead of clicking them (since they aren't clickable links).
