That's what the article describes, but it's hard to validate an email address by sending to it too if you want time bounds. My mail server implements greylistin

That's what the article describes, but it's hard to validate an email address by sending to it too if you want time bounds. My mail server implements greylisting and its frequently difficult for me to verify my email address on services that send me tokens that expire. Greylisting typically delays by only 10 minutes or so but there are plenty of times that mail servers can be down for extended periods, or a quota/disc is full, or an intermediate mail router is down, or any number of other problems