Mildly Infuriating

There you have it.

When I’ve been in OP’s situation, I filed a complaint with the FCC, performed a whois lookup on their site to send emails to the abuse/spam emails of their DNS registrar and host and inspected the email headers to email their email provider’s abuse/spam account(s). I’ve not yet had cause to reach out to my attorney general’s office when I’ve had a company violate CAN-SPAM, but it’s an option.
I also make sure each company knows there’s a pending CAN-SPAM complaint. I keep it convivial, but serious. “Hey, just letting you know that one of your clients is violating your terms of service and the law! A complaint has already been lodged with the FCC. Toodeloo!”
That bit of knowledge tends to shift the interpretation of your complaint from “annoyed nerd” to “someone politely informing you that you’re going to get skull fucked by the long dick of the law if you don’t fix this ASAP”

It may sound sort of excessive, but I’m a bit of a consumer rights absolutist.

Are single page apps considered one page?

With a lot of people using free email services, most have some report button. What this does is flag the specific email as potential spam that you specifically do not want to see. With enough people doing that, the probability of the email and subsequently the source domain being spam and spam generators goes up. High probability means the emails may end up in the spam folder without hitting your inbox.

There's a bit of fine tuning email marketing can do to mitigate that, like not sending emails too frequently. But that's not a passive thing they can do, which is why there are teams devoted to email marketing specifically at some companies.

The worst thing for a marketeer is to be dumped in spam. No one will ever see it or any future emails.

Yeah I had to do this to a couple lists because I have a very simple email address that gets added to things all the time, but if it's really irritating to unsubscribe, I just click my email settings to report spam or fishing, and that usually creates an automatic filter for that center so you never get bothered again.

You can manually create a filter in the settings to send all their messages to trash or spam if you want as well.

A lot of services won’t let you use the +, and it’s also trivial to get rid of the extension with Regex.

You can also create aliases on most sites by adding a + and a suffix to your email

You can never disable those aliases, though. The best you can do is write a filter that sends them to trash. With a good email alias system, you can actually disable an alias, so that emails to it just bounce back.

Many services just strip out everything at the + so I instead have a catchall account use email addresses like [email protected]. For addresses that start getting spam, I add them to a config in rspamd that bounces them. (I self-host my email server)