On 2025-05-08 08:11:18, Marc wrote: > > I used a greylist where emails get a 4xx message with a link that > allows the email through so 'regular' users can get past it. Problem > is, these fucked up networks are not even notifying users about 5xx > and 4xx notifications.
With sendgrid, it depends on the 4xx message, but they often just silently delete the message. It turns out that, even though the RFCs explicitly forbid you from doing it, this multi-billion dollar company has a list of hand-written regexes that they use to "parse" each rejection message. If the regex decides that a retry is unlikely to succeed, they delete it. Most interestingly, they apparently delete messages that get a connection refused. In the past this was easy to test: turn off your mail server, do something on Github, turn it back on. The notifications would never arrive. They're a spam company whose massive marketing department has tricked people into thinking they're good for sending transactional email. But caveat emptor, they treat your password resets and order receipts like spam too.
