Emails are being rejected for SPAM
IP address from iCloud is listed on many RBLs (Reputable Black Lists). Spam filters are rejecting my email
iPhone 15, iOS 17
Question marked as
Are you sure it's the iCloud IP address and not your device's? I'd be surprised if it is because there would be millions of people with problems. Try sending mails from a mobile device with wireless turned off to confirm and if they are still rejected then you need to contact Apple directly. This is a user-to-user board. Apple support can be found here.
If the mail from the mobile device goes through OK then it's your IP. Unless you have your own personal fixed IP address then turn your router off for a couple of minutes and turn it on again. If the problem still persists then you need to contact your ISP - although they'll probably be aware of it.