[necovek]

One of the problems is the pure mechanics of how SMTP works: anyone can send an email with any "from" address, and filtering on people/domains in the "from" field is basically pointless if you want to catch spam.

What Google is likely doing is checking the domain of the originating IP in the SMTP "envelope", but that also gets tricky with outsourced email services or internal IPs.

I do not use Gmail, so these are just wild guesses, but I do run my own mail server and frequently get my email not showing up for people.

Problem with SMTP bounces is that it may take a week for the final bounce to show up in my inbox (because again, that's how SMTP protocol is designed, to expect nodes to be down and retry a number of times).

[wtfrmyinitials]

> anyone can send an email with any "from" address, and filtering on people/domains in the "from" field is basically pointless if you want to catch spam.

Isn’t this the exact thing that DKIM is designed to fix?

[necovek]

This was a response to a complaint about not receiving emails from some "people/domains" even when whitelisted, and I highlighted how this can be hard for a service to detect reliably.

So yes, DKIM will help a receiving server know for sure, but a receiving server still needs to accept emails from non-DKIM-enabled servers, and perhaps that's why whitelisting didn't work for the parent.

[Neil44]

Bounce handling depends on the error code, 5xx is usually a permanent error so the bounce is immediate. Mailbox doesn't exist etc. Its not going to exist later either. IIRC 4xx is effectively try again later because something is wrong right now that might get fixed. Mail server out of disk space, you're greylisted etc. So try again later. Those can take a few days to give up retrying.