[comrade1234]

Wanted to add something... the biggest issue I've had in the last few years is having to whitelist senders to skip the temporary rejection (smtp 4xx code) process to reduce spam.

Basically, someone new emails me and my server responds with a temporary rejection message saying to try again later and then when their server retries the message 5 minutes later it allows it to go through. This is a standard process to block spammers.

However lately when their email service resends the message it will come from a different server. Something like mailserver-1, then mailserver-2, then mailserver-3, each with a different ip address and each time it gets rejected until it reuses one of the addresses. But with apple for example they have hundreds of servers to cycle through and eventually the message times out and is rejected at their end.

So I have to whitelist senders to skip the temporary rejection. It comes up every few months for me, having to whitelist someone. I think it's a result of every small- mid-size company moving to providers and not hosting their own email, and these providers having dozens of servers. The domain of the server never matches the domain of the sender...