It's been about 99% fine. I do remember last year noticing that my parent's gmail flagged (an admittedly very short) email of mine as spam, but when she went into her spam folder and told gmail it wasn't spam, all was fine from then on. I think it probably goes a long way to have SPF/DKIM/DMARC correct and doesn't hurt that it's AWS' mail servers.
iCloud email has been part of some sketchy things in the past, supposedly they have been dropping emails with certain words in them with no notification to either side of the message. There isn’t an option to turn it off and it isn’t advertised anywhere https://forums.techsoup.org/t/icloud-email-system-blocks-ema...
Your link is from over 10 years ago. I can't find any newer information on it persisting. Perhaps they turned it off after they got caught? Or perhaps it was an outage or some thing and not actual filtering? I don't know.
All email providers have issues now and again.
I'm not saying they aren't doing it, but if they are, nobody seems to be complaining or upset about it except those few in that forum from over a decade ago.
Resend just kept getting stuck in spam folders for me when sending, maybe I set something up wrong. On their documentation it just says you have to pay for the extremely expensive certificate needed for gmail to trust your messages.
Often it's just a shared IP address that is naughty, you can usually raise a ticket and ask to transfer to a new pool. There is no "expensive certificate" you can buy to bypass spam filters and anyone selling you that is selling snake oil.
The founder of foundermail actually left a comment here talking about this. I will likely update the article that it took less than 7 hours to be approved.
