[blablabla123]

Just from the user perspective, frequently legit E-mail goes into my spam folder. This happens with different major E-Mail providers. Although it's mail I'm not interested in, it gets flagged as spam and I unflag it if I find the time. But the point is, usually it's double-opted in mailings which is not interesting to me at all and received in much to high frequency.

So if you are writing a system for that from scratch, consider decreasing frequency for people who don't open the mail. So it passes through spam filters, and maybe raises attention of readers, just for curiosity...

[ghaff]

A huge amount of not-really-spam goes to my Gmail spam folder. It's mostly email I at least implicitly agreed to receive by dint of attending a webinar, downloading a report, getting scanned at a conference, or even explicitly signing for a newsletter at some point.

I know some people would rather none of this email existed, but it does and it's how, in the real world, it's part of how customers are acquired and kept. The legit mail of this type all has obvious unsubscribe links but I'm sure a lot of people just flag it as spam instead and, over some threshold, it starts to go to others' spam folders too.