Lately, I had a case when someone informed me that he would take my e-mail from his newsletter mailing list because I seemed to be a dead address (and every address generated cost for him). I replied that I did not like to be tracked (implying that I had read the newsletter) and we had a nice conversation. Not every fake e-mail is as fake as it seems ;-)
Apparently, Thunderbird can be quite good at beating E-Mail Tracking.
If your free users are using up paid user resources, that's a different problem.
I could tell because there were emails with the sequence of:
1@xyz.com 2@xyz.com ... etc
Are there any "privacy-first" SSO options? Maybe Proton or Mullvad or some similarly positioned company should think about this. Or maybe it's just way too hard to break into the Google/Apple duopoly.
If you’re going to offer SSO, at least take the lowly amount of time to add some other providers. I genuinely don’t believe anyone is launching so quickly that they don’t have time to add a second or third option.