Why is IP reputation important?

I dont understand the importance of IP reputation. It seems like an outdated concept to me like the 3 step TCP handshake (vs QUIC). I think domain reputation is all that should matter because who cares which server is sending mail as long as its authentic and from a reputation domain.

Can someone explain?

Because it would be easier to sent malicious emails if it’s just looking at the domain. Getting a clean domain is not that hard—get a reputable TLD, put some content up, and you’re done. And once you get flagged, buy a new domain, get a new certificate and switch the configs out.
Looking at the domain and IP makes it more “difficult” to get a cleansheet.

For QUIC, many systems still rely on TCP/IP for communication, which makes IP reputation still relevant

You can switch out the IP of your mail server just as easily as the TLD. QUIC is a new technology, how older HTTP is implemented has nothing to do with mail server IP reputation.

Of course, nothing online is foolproof. Getting a new IP address and domain just makes it more difficult, like I said. At some point you could also run out of IPs due to stock or just lack of financial capability. If you want to do something with malicious intent, it’s always possible.

How older HTTP is implemented is true, unless specific security vulnerabilities or misconfigurations associated with that technology could affect the server’s behavior.