Lest anyone think Gmail is less susceptible to problems because of the sheer volume of mail they handle (so any problem gets a lot of 'eyes' on it), they had an

Lest anyone think Gmail is less susceptible to problems because of the sheer volume of mail they handle (so any problem gets a lot of 'eyes' on it), they had an amusing and long-standing bug that meant any email containing a link to any domain name starting with "0x" automatically went to spam. You could literally have two long standing, legitimately used accounts send an email to each other containing a link to a URL like http://0xANYTHINGHERE.com/ http://0xANYTHINGHERE.com/ and it would be insta-spammed. I suspect it was a hard coded rule to avoid people using "long IP" URLs to circumvent other filters.. except there are lots of legitimate 0x domains that aren't long IPs. It was fixed sometime in the past year but I got a lot of use out of it in talks I've given about email deliverability over the years.