Hi The Nipponese,
We understand you are unable to reply to email received when using "Hide My Email". "In order to see Hide My Email messages in Mail on iCloud.com, set the forwarding email address to your iCloud Mail address or a custom domain address". Follow the steps and article below for responding.
- "In Mail on iCloud.com, select the message in your message list, then tap the Reply, Reply All, Forward button.
- Tap Reply"
Use Hide My Email in Mail on iCloud.com
If you’re still having an issue using Hide My Email, please make sure your iPhone is up to date. The current iOS version is iOS 16.0.2. You can see which iOS version your iPhone is on by going to Settings > General > About. Update your iPhone or iPad - Apple Support
Have a nice day.